Optical Systems Having Polarization Recycling Structures

ABSTRACT

A display may include illumination optics, a ferroelectric liquid crystal on silicon (fLCOS) panel, and a waveguide. The illumination optics may produce linear polarized illumination light that is modulated by the fLCOS panel to produce image light. The illumination optics may include light emitters that emit respective wavelengths of the illumination light. The illumination optics may include an X-plate that outputs the illumination light by combining the light emitted by the light emitters. Polarization recycling structures may be optically interposed between each of the light emitters and the X-plate. The polarization recycling structures may include a reflective polarizer. If desired, the polarizing recycling structures may also include a quarter waveplate. The polarization recycling structures may serve to minimize the amount of light lost in producing linearly polarized illumination light for the fLCOS display panel, thereby maximizing the optical efficiency of the display.

This application is a continuation of international patent applicationNo. PCT/US2021/047620, filed Aug. 25, 2021, which claims priority toU.S. provisional patent application No. 63/071,996, filed Aug. 28, 2020,which are hereby incorporated by reference herein in their entireties.

BACKGROUND

This relates generally to optical systems and, more particularly, tooptical systems for displays.

Electronic devices may include displays that present images to a user’seyes. For example, devices such as virtual reality and augmented realityheadsets may include displays with optical elements that allow users toview the displays.

It can be challenging to design devices such as these. If care is nottaken, the components used in displaying content may be unsightly andbulky, can consume excessive power, and may not exhibit desired levelsof optical performance.

SUMMARY

An electronic device such as a head-mounted device may have one or morenear-eye displays that produce images for a user. The head-mounteddevice may be a pair of virtual reality glasses or may be an augmentedreality headset that allows a viewer to view both computer-generatedimages and real-world objects in the viewer’s surrounding environment.

The display may include a display module and a waveguide. The displaymodule may include a spatial light modulator such as a ferroelectricliquid crystal on silicon (fLCOS) display panel and illumination optics.The illumination optics may include light sources such as light emittingdiodes (LEDs) that produce illumination light. The illumination lightmay be provided with a linear polarization and may be transmitted to thefLCOS display panel. The fLCOS display panel may modulate image data(e.g., image frames) onto the illumination light to produce image light.The waveguide may direct the image light towards an eye box.

The illumination optics may include at least first and second lightemitters that emit respective wavelengths of the illumination light. Theillumination optics may include an X-plate that outputs the illuminationlight by combining the light emitted by the first and second lightemitters. Polarization recycling structures may be optically interposedbetween each of the light emitters and the X-plate. The polarizationrecycling structures for a given one of the light emitters may include areflective polarizer. If desired, the polarizing recycling structuresmay also include a quarter waveplate optically interposed between thereflective polarizer and the light emitter. The polarization recyclingstructures may serve to minimize the amount of light lost in producinglinearly polarized illumination light for the fLCOS display panel,thereby maximizing the optical efficiency of the display. Polarizing theillumination light prior to the X-plate may also optimize the spectralperformance of the illumination light, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative system having a display inaccordance with some embodiments.

FIG. 2 is a top view of an illustrative optical system for a displayhaving a display module that provides image light to a waveguide inaccordance with some embodiments.

FIG. 3 is a top view of an illustrative display module having aferroelectric liquid crystal on silicon (fLCOS) display panel inaccordance with some embodiments.

FIG. 4 is a cross-sectional side view of an illustrative fLCOS displaypanel in accordance with some embodiments.

FIG. 5 is a plot of ferroelectric liquid crystal (fLC) efficiency as afunction of cell gap for an illustrative fLCOS display panel inaccordance with some embodiments.

FIG. 6 is a top view of an illustrative light source having polarizationrecycling structures with a reflective polarizer in accordance with someembodiments.

FIG. 7 is a top view of an illustrative light source having polarizationrecycling structures with a reflective polarizer and a quarter waveplate in accordance with some embodiments.

FIG. 8 is a plot of optical performance (luminance as a function ofincident angle) for a light source having different polarizationrecycling structures in accordance with some embodiments.

FIG. 9 is a plot of optical performance (efficiency improvement as afunction of integrating cone angle) for a light source havingpolarization recycling structures of the types shown in FIGS. 6 and 7 inaccordance with some embodiments.

FIG. 10 is a cross-sectional side view of an illustrative light sourcehaving integral polarization recycling structures in accordance withsome embodiments.

FIGS. 11 and 12 are a cross-sectional side views of an illustrativelight source having polarization recycling structures separated from theemissive area of the light source by an air gap in accordance with someembodiments.

FIG. 13 is a cross-sectional side view of an illustrative light sourcehaving integral polarization recycling structures on a ceramic substratein accordance with some embodiments.

FIG. 14 is a cross-sectional side view showing how illustrativepolarization recycling structures may be shared by multiple lightsources in accordance with some embodiments.

FIGS. 15 and 16 are cross-sectional side views of an illustrative lightsource having polarization recycling structures integrated with acondenser lens in accordance with some embodiments.

FIG. 17 is a top view of an illustrative X-plate that may be providedwith interference coatings for reflecting and transmitting light fromlight sources in accordance with some embodiments.

FIG. 18 is a plot showing how polarization recycling structures mayoptimize optical performance (X-plate reflection as a function ofwavelength) for light sources in a display module in accordance withsome embodiments.

FIG. 19 is a timing diagram of illustrative illumination sequences thatmay be used by light sources to optimize power consumption in a displaymodule in accordance with some embodiments.

FIG. 20 is a flow chart of illustrative steps that may be involved incontrolling an fLCOS display panel to display images based on agreen-heavy illumination sequence in accordance with some embodiments.

FIG. 21 is a flow chart of illustrative steps that may be involved incontrolling light sources using a green-heavy illumination sequence inaccordance with some embodiments.

FIG. 22 is a flow chart of illustrative steps for driving an fLCOSdisplay panel to compensate for chromatic aberrations in a displaymodule in accordance with some embodiments.

FIG. 23 is a CIE1931 color space plot that shows how illuminating anfLCOS panel using an illustrative green-heavy illumination sequence maymodify the color gamut for images produced by the fLCOS panel inaccordance with some embodiments.

FIG. 24 is a top view of an illustrative display having spatial pixelshifting structures that increase the effective resolution of imagesprovided at an eye box in accordance with some embodiments.

FIG. 25 is a top view of an illustrative display having angular pixelshifting structures that increase the effective resolution of imagesprovided at an eye box in accordance with some embodiments.

FIG. 26 is a front view of pixels of image light that illustrates howillustrative pixel shifting structures the types shown in FIGS. 24 and25 may increase the effective resolution of the image light inaccordance with some embodiments.

FIG. 27 is a timing diagram of illustrative driving voltages that may beused to drive an fLCOS display panel in accordance with someembodiments.

FIG. 28 is a timing diagram showing how an illustrative fLCOS displaypanel may be overdriven by a non-square wave driving voltage waveform inaccordance with some embodiments.

FIG. 29 is a flow chart of illustrative steps that may be involved inoverdriving an fLCOS display panel based on temperature sensormeasurements in accordance with some embodiments.

FIG. 30 is a flow chart of illustrative steps that may be involved inoverdriving an fLCOS display panel based on frame history information inaccordance with some embodiments.

FIG. 31 is a plot of fLCOS performance (response time as a function oftemperature) that shows how overdriving an fLCOS display panel based ontemperature sensor measurements may minimize fLCOS response time inaccordance with some embodiments.

DETAILED DESCRIPTION

An illustrative system having a device with one or more near-eye displaysystems is shown in FIG. 1 . System 10 may be a head-mounted devicehaving one or more displays such as near-eye displays 14 mounted withinsupport structure (housing) 20. Support structure 20 may have the shapeof a pair of eyeglasses (e.g., supporting frames), may form a housinghaving a helmet shape, or may have other configurations to help inmounting and securing the components of near-eye displays 14 on the heador near the eye of a user. Near-eye displays 14 may include one or moredisplay modules such as display modules 14A and one or more opticalsystems such as optical systems 14B. Display modules 14A may be mountedin a support structure such as support structure 20. Each display module14A may emit light 22 (sometimes referred to herein as image light 22)that is redirected towards a user’s eyes at eye box 24 using anassociated one of optical systems 14B.

The operation of system 10 may be controlled using control circuitry 16.Control circuitry 16 may include storage and processing circuitry forcontrolling the operation of system 10. Circuitry 16 may include storagesuch as hard disk drive storage, nonvolatile memory (e.g.,electrically-programmable-read-only memory configured to form a solidstate drive), volatile memory (e.g., static or dynamicrandom-access-memory), etc. Processing circuitry in control circuitry 16may be based on one or more microprocessors, microcontrollers, digitalsignal processors, baseband processors, power management units, audiochips, graphics processing units, application specific integratedcircuits, and other integrated circuits. Software code (instructions)may be stored on storage in circuitry 16 and run on processing circuitryin circuitry 16 to implement operations for system 10 (e.g., datagathering operations, operations involving the adjustment of componentsusing control signals, image rendering operations to produce imagecontent to be displayed for a user, etc.).

System 10 may include input-output circuitry such as input-outputdevices 12. Input-output devices 12 may be used to allow data to bereceived by system 10 from external equipment (e.g., a tetheredcomputer, a portable device such as a handheld device or laptopcomputer, or other electrical equipment) and to allow a user to providehead-mounted device 10 with user input. Input-output devices 12 may alsobe used to gather information on the environment in which system 10(e.g., head-mounted device 10) is operating. Output components indevices 12 may allow system 10 to provide a user with output and may beused to communicate with external electrical equipment. Input-outputdevices 12 may include sensors and other components 18 (e.g., imagesensors for gathering images of real-world object that are digitallymerged with virtual objects on a display in system 10, accelerometers,depth sensors, light sensors, haptic output devices, speakers,batteries, wireless communications circuits for communicating betweensystem 10 and external electronic equipment, etc.). In one suitablearrangement that is sometimes described herein as an example, thesensors in components 18 may include one or more temperature (T) sensors19. Temperature sensor(s) 19 may gather temperature sensor data (e.g.,temperature values) from one or more locations in system 10. If desired,control circuitry 16 may use the gathered temperature sensor data incontrolling the operation of display module 14A.

Display modules 14A (sometimes referred to herein as display engines14A, light engines 14A, or projectors 14A) may include reflectivedisplays (e.g., displays with a light source that produces illuminationlight that reflects off of a reflective display panel to produce imagelight such as liquid crystal on silicon (LCOS) displays (e.g.,ferroelectric liquid crystal on silicon (fLCOS) displays),digital-micromirror device (DMD) displays, or other spatial lightmodulators), emissive displays (e.g., micro-light-emitting diode (uLED)displays, organic light-emitting diode (OLED) displays, laser-baseddisplays, etc.), or displays of other types. An arrangement in whichdisplay module 14A includes an fLCOS display is sometimes describedherein as an example. Light sources in display modules 14A may includeuLEDs, OLEDs, LEDs, lasers, combinations of these, or any other desiredlight-emitting components.

Optical systems 14B may form lenses that allow a viewer (see, e.g., aviewer’s eyes at eye box 24) to view images on display(s) 14. There maybe two optical systems 14B (e.g., for forming left and right lenses)associated with respective left and right eyes of the user. A singledisplay 14 may produce images for both eyes or a pair of displays 14 maybe used to display images. In configurations with multiple displays(e.g., left and right eye displays), the focal length and positions ofthe lenses formed by components in optical system 14B may be selected sothat any gap present between the displays will not be visible to a user(e.g., so that the images of the left and right displays overlap ormerge seamlessly).

If desired, optical system 14B may contain components (e.g., an opticalcombiner, etc.) to allow real-world image light from real-world imagesor objects 25 to be combined optically with virtual (computer-generated)images such as virtual images in image light 22. In this type of system,which is sometimes referred to as an augmented reality system, a user ofsystem 10 may view both real-world content and computer-generatedcontent that is overlaid on top of the real-world content. Camera-basedaugmented reality systems may also be used in device 10 (e.g., in anarrangement in which a camera captures real-world images of object 25and this content is digitally merged with virtual content at opticalsystem 14B).

System 10 may, if desired, include wireless circuitry and/or othercircuitry to support communications with a computer or other externalequipment (e.g., a computer that supplies display 14 with imagecontent). During operation, control circuitry 16 may supply imagecontent to display 14. The content may be remotely received (e.g., froma computer or other content source coupled to system 10) and/or may begenerated by control circuitry 16 (e.g., text, other computer-generatedcontent, etc.). The content that is supplied to display 14 by controlcircuitry 16 may be viewed by a viewer at eye box 24.

FIG. 2 is a top view of an illustrative display 14 that may be used insystem 10 of FIG. 1 . As shown in FIG. 2 , display 14 may include one ormore display modules such as display module 14A and an optical systemsuch as optical system 14B. Optical system 14B may include opticalelements such as one or more waveguides 26. Waveguide 26 may include oneor more stacked substrates (e.g., stacked planar and/or curved layerssometimes referred to herein as waveguide substrates) of opticallytransparent material such as plastic, polymer, glass, etc.

If desired, waveguide 26 may also include one or more layers ofholographic recording media (sometimes referred to herein as holographicmedia, grating media, or diffraction grating media) on which one or morediffractive gratings are recorded (e.g., holographic phase gratings,sometimes referred to herein as holograms). A holographic recording maybe stored as an optical interference pattern (e.g., alternating regionsof different indices of refraction) within a photosensitive opticalmaterial such as the holographic media. The optical interference patternmay create a holographic phase grating that, when illuminated with agiven light source, diffracts light to create a three-dimensionalreconstruction of the holographic recording. The holographic phasegrating may be a non-switchable diffractive grating that is encoded witha permanent interference pattern or may be a switchable diffractivegrating in which the diffracted light can be modulated by controlling anelectric field applied to the holographic recording medium. Multipleholographic phase gratings (holograms) may be recorded within (e.g.,superimposed within) the same volume of holographic medium if desired.The holographic phase gratings may be, for example, volume holograms orthin-film holograms in the grating medium. The grating media may includephotopolymers, gelatin such as dichromated gelatin, silver halides,holographic polymer dispersed liquid crystal, or other suitableholographic media.

Diffractive gratings on waveguide 26 may include holographic phasegratings such as volume holograms or thin-film holograms, meta-gratings,or any other desired diffractive grating structures. The diffractivegratings on waveguide 26 may also include surface relief gratings formedon one or more surfaces of the substrates in waveguides 26, gratingsformed from patterns of metal structures, etc. The diffractive gratingsmay, for example, include multiple multiplexed gratings (e.g.,holograms) that at least partially overlap within the same volume ofgrating medium (e.g., for diffracting different colors of light and/orlight from a range of different input angles at one or morecorresponding output angles).

Optical system 14B may include collimating optics such as collimatinglens 34. Collimating lens 34 may include one or more lens elements thathelp direct image light 22 towards waveguide 26. Collimating lens 34 isshown external to display module 14A in FIG. 2 for the sake of clarity.In general, collimating lens 34 may be formed entirely external todisplay module 14A, entirely within display module 14A, or one or morelens elements in collimating lens 34 may be formed in display module 14A(e.g., collimating lens 34 may include both lens elements that areinternal to display module 14A and lens elements that are external todisplay module 14A). Collimating lens 34 may be omitted if desired. Ifdesired, display module(s) 14A may be mounted within support structure20 of FIG. 1 while optical system 14B may be mounted between portions ofsupport structure 20 (e.g., to form a lens that aligns with eye box 24).Other mounting arrangements may be used, if desired.

As shown in FIG. 2 , control circuitry 16 may control display module 14Ato generate image light 22 associated with image content (data) to bedisplayed to (at) eye box 24. In the example of FIG. 2 , display module14A includes illumination optics 36 and a spatial light modulator suchas fLCOS display panel 40 (sometimes referred to herein simply as fLCOSpanel 40).

Control circuitry 16 may be coupled to illumination optics 36 overcontrol path(s) 42. Control circuitry 16 may be coupled to fLCOS panel40 over control path(s) 44. Control circuitry 16 may provide controlsignals to illumination optics 36 over control path(s) 42 that controlillumination optics 36 to produce illumination light 38 (sometimesreferred to herein as illumination 38). The control signals may, forexample, control illumination optics 36 to produce illumination light 38using a corresponding illumination sequence. The illumination sequencemay involve sequentially illuminating light sources of different colorsin illumination optics 36. In one suitable arrangement that is sometimesdescribed herein as an example, the illumination sequence may be agreen-heavy illumination sequence.

Illumination optics 36 may illuminate fLCOS display panel 40 usingillumination light 38. Control circuitry 16 may provide control signalsto fLCOS display panel 40 over control path(s) 44 that control fLCOSdisplay panel 40 to modulate illumination light 38 to produce imagelight 22. For example, control circuitry 16 may provide image data suchas image frames to fLCOS display panel 40. The image light 22 producedby fLCOS display panel 40 may include the image frames identified by theimage data. Control circuitry 16 may, for example, control fLCOS displaypanel 40 to provide fLCOS drive voltage waveforms to electrodes in thedisplay panel. The fLCOS drive voltage waveforms may be overdriven orunderdriven to optimize the performance of display module 14A, ifdesired. While an arrangement in which display module 14A includes fLCOSdisplay panel 40 is described herein as an example, in general, displaymodule 14A may include any other desired type of reflective displaypanel (e.g., a DMD panel), an emissive display panel, etc.

Image light 22 may be collimated using collimating lens 34 (sometimesreferred to herein as collimating optics 34). Optical system 14B may beused to present image light 22 output from display module 14A to eye box24. Optical system 14B may include one or more optical couplers such asinput coupler 28, cross-coupler 32, and output coupler 30. In theexample of FIG. 2 , input coupler 28, cross-coupler 32, and outputcoupler 30 are formed at or on waveguide 26. Input coupler 28,cross-coupler 32, and/or output coupler 30 may be completely embeddedwithin the substrate layers of waveguide 26, may be partially embeddedwithin the substrate layers of waveguide 26, may be mounted to waveguide26 (e.g., mounted to an exterior surface of waveguide 26), etc.

The example of FIG. 2 is merely illustrative. One or more of thesecouplers (e.g., cross-coupler 32) may be omitted. Optical system 14B mayinclude multiple waveguides that are laterally and/or vertically stackedwith respect to each other. Each waveguide may include one, two, all, ornone of couplers 28, 32, and 30. Waveguide 26 may be at least partiallycurved or bent if desired.

Waveguide 26 may guide image light 22 down its length via total internalreflection. Input coupler 28 may be configured to couple image light 22from display module(s) 14A into waveguide 26 (e.g., at an angle suchthat the image light can propagate down waveguide 26 via total internalreflection), whereas output coupler 30 may be configured to couple imagelight 22 from within waveguide 26 to the exterior of waveguide 26 andtowards eye box 24. Input coupler 28 may include a reflective ortransmissive input coupling prism if desired. As an example, displaymodule(s) 14A may emit image light 22 in the +Y direction towardsoptical system 14B.

When image light 22 strikes input coupler 28, input coupler 28 mayredirect image light 22 so that the light propagates within waveguide 26via total internal reflection towards output coupler 30 (e.g., in the +Xdirection). When image light 22 strikes output coupler 30, outputcoupler 30 may redirect image light 22 out of waveguide 26 towards eyebox 24 (e.g., back in the -Y direction). In scenarios wherecross-coupler 32 is formed at waveguide 26, cross-coupler 32 mayredirect image light 22 in one or more directions as it propagates downthe length of waveguide 26, for example. In this way, display module 14Amay provide image light 22 to eye box 24 over an optical path thatextends from display module 14A, through collimating lens 34, inputcoupler 28, cross coupler 32, and output coupler 30.

Input coupler 28, cross-coupler 32, and/or output coupler 30 may bebased on reflective and refractive optics or may be based on holographic(e.g., diffractive) optics. In arrangements where couplers 28, 30, and32 are formed from reflective and refractive optics, couplers 28, 30,and 32 may include one or more reflectors (e.g., an array ofmicromirrors, partial mirrors, louvered mirrors, or other reflectors).In arrangements where couplers 28, 30, and 32 are based on holographicoptics, couplers 28, 30, and 32 may include diffractive gratings (e.g.,volume holograms, surface relief gratings, etc.).

FIG. 3 is a top view of display module 14A. As shown in FIG. 3 , displaymodule 14A may include illumination optics 36 that provide illuminationlight 38 to fLCOS display panel 40. fLCOS display panel 40 may modulateimages onto illumination light 38 to produce image light 22.

Illumination optics 36 may include one or more light sources 48 such asa first light source 48A, a second light source 48B, and a third lightsource 48C. Light sources 48 may emit illumination light 52. Prism 46(e.g., an X-plate) in illumination optics 36 may combine theillumination light 52 emitted by each of the light sources 48 to producethe illumination light 38 that is provided to fLCOS display panel 40. Inone suitable arrangement that is sometimes described herein as anexample, first light source 48A emits red illumination light 52A (e.g.,light source 48A may be a red (R) light source), second light source 48Bemits green illumination light 52B (e.g., light source 48B may be agreen (G) light source), and third light source 48C emits blueillumination light 52C (e.g., light source 48C may be a blue (B) lightsource). This is merely illustrative. In general, light sources 48A,48B, and 48C may respectively emit light in any desired wavelength bands(e.g., visible wavelengths, infrared wavelengths, near-infraredwavelengths, etc.).

An arrangement in which illumination optics 36 includes only one lightsource 48A, one light source 48B, and one light source 48C is sometimesdescribed herein as an example. This is merely illustrative. If desired,illumination optics 36 may include any desired number of light sources48A (e.g., an array of light sources 48A), any desired number of lightsources 48B (e.g., an array of light sources 48B), and any desirednumber of light sources 48C (e.g., an array of light sources 48C). Lightsources 48A, 48B, and 48C may include LEDs, OLEDs, uLEDs, lasers, or anyother desired light sources. An arrangement in which light sources 48A,48B, and 48C are LED light sources is described herein as an example.Light sources 48A, 48B, and 48C may be controlled (e.g.,separately/independently controlled) by control signals received fromcontrol circuitry 16 (FIG. 2 ) over control path(s) 42. The controlsignals may, for example, control light sources 48A, 48B, and 48C toemit illumination light 52 using a corresponding illumination sequencein which one or more of the light sources emits illumination light atany given time and the active light sources cycle over time.

Illumination light 38 may include the illumination light 52A, 52B, and52C emitted by light sources 48A, 48B, and 48C, respectively. Prism 50may provide illumination light 38 to fLCOS display panel 40. If desired,additional optical components such as lens elements, microlenses,polarizers, prisms, beam splitters, and/or diffusers (not shown in FIG.3 for the sake of clarity) may be optically interposed between lightsources 48A-C and fLCOS display panel 40 to help direct illuminationlight 38 from illumination optics 36 to fLCOS display panel 40.

Prism 50 may direct illumination light 38 onto fLCOS display panel 40(e.g., onto different pixels P* on fLCOS display panel 40). Controlcircuitry 16 may provide control signals to fLCOS display panel 40 overcontrol path(s) 44 that control fLCOS display panel 40 to selectivelyreflect illumination light 38 at each pixel location to produce imagelight 22 (e.g., image light having an image as modulated onto theillumination light by fLCOS display panel 40). As an example, thecontrol signals may drive fLCOS drive voltage waveforms onto the pixelsof fLCOS display panel 40. Prism 50 may direct image light 22 towardscollimating lens 34 of FIG. 2 .

In general, fLCOS display panel 40 operates on illumination light of asingle linear polarization. Polarizing structures interposed on theoptical path between light sources 48A-C and fLCOS display panel 40 mayconvert unpolarized illumination light into linearly polarizedillumination light (e.g., s-polarized light or p-polarized illuminationlight). The polarizing structures may, for example, be opticallyinterposed between prism 50 and fLCOS display panel 40, between prism 46and prism 50, between light sources 48A-C and prism 46, within lightsources 48A-C, or elsewhere.

If a given pixel P* in fLCOS display panel 40 is turned on, thecorresponding illumination light may be converted between linearpolarizations by that pixel of the display panel. For example, ifs-polarized illumination light 38 is incident upon a given pixel P*,fLCOS display panel 40 may reflect the s-polarized illumination light 38to produce corresponding image light 22 that is p-polarized when pixelP* is turned on. Similarly, if p-polarized illumination light 38 isincident upon pixel P*, fLCOS display panel 40 may reflect thes-polarized illumination light 38 to produce corresponding image light22 that is s-polarized when pixel P* is turned on. If pixel P* is turnedoff, the pixel does not convert the polarization of the illuminationlight, which prevents the illumination light from reflecting out offLCOS display panel 40 as image light 22.

FIG. 4 is a cross-sectional side view of fLCOS display panel 40. Fourpixels P* in fLCOS display panel 40 are illustrated in FIG. 4 for thesake of clarity. In general, fLCOS display panel 40 may include anydesired number of pixels P* arranged in any desired pattern (e.g., anydesired number of rows and columns).

As shown in FIG. 4 , fLCOS display panel 40 may include a flexibleprinted circuit 74 (sometimes referred to herein as driver flex 74).Driver flex 74 may be layered onto substrate 76. This is merelyillustrative and, if desired, substrate 76 may be omitted. Driver flex74 may carry control path(s) 44 (FIG. 2 ) for driving the pixels P* infLCOS display panel 40, for example.

A backplane such as backplane 72 may be layered over driver flex 74.Backplane 72 may serve as a reflective surface for reflecting incidentillumination light 38 as corresponding image light 22. In somescenarios, backplane 72 is an aluminum backplane made from aluminummetal. However, in practice, forming backplane 72 from aluminum maylimit the overall reflective performance of fLCOS display panel 40,thereby limiting the overall optical performance and efficiency ofdisplay module 14A.

In order to increase the reflectivity of backplane 72, backplane 72 maybe formed from silver or a silver alloy (e.g., backplane 72 may be asilver backplane or a silver alloy backplane). Forming backplane 72 fromsilver may, for example, increase the amount of reflection in media forfLCOS display panel 40 from around 86% (in scenarios where backplane 72is formed from aluminum) to as high as around 97%. Forming backplane 72from silver alloy may optimize the stability of the system, for example.In another suitable arrangement, backplane 72 may be a dielectric mirrorbackplane. Forming backplane 72 from a dielectric mirror may alsoincrease the reflectance of fLCOS display panel 40 relative to scenarioswhere an aluminum backplane is used.

An alignment layer such as polyimide alignment layer 70 may be layeredover backplane 72. A ferroelectric liquid crystal (fLC) layer such asfLC layer 68 may be layered over polyimide alignment layer 70. Anadditional polyimide alignment layer 66 may be layered over fLC layer68. Polyimide alignment layers 70 and 66 may, for example, serve toalign the fLC molecules in fLC layer 68 at the upper and lower surfacesof fLC layer 68.

An electrode layer such as electrode layer 64 may be layered overpolyimide alignment layer 66. Electrode layer 64 may include indium tinoxide (ITO) traces or index-matching indium tin oxide (IMITO) traces, asexamples. Electrode layer 64 may, for example, receive fLCOS drivevoltage waveforms that control the state of each pixel P* in fLCOSdisplay panel 40 (e.g., to reflect incident illumination light 38 of afirst polarization as corresponding image light 22 of a secondpolarization when the pixel is turned on and to reflect illuminationlight 38 with the first polarization when the pixel is turned off,thereby preventing the reflected light from passing to waveguide 26 ofFIG. 2 as image light 22).

A cover layer such as cover glass 62 may be layered over electrode layer64 (e.g., electrode layer 64 may be patterned onto the lower surface ofcover glass 62). An optional anti-reflective coating 60 may be layeredover cover glass 62 to minimize reflections at the upper surface ofcover glass 62. As shown in FIG. 4 , illumination light 38 may passthrough anti-reflective coating 60, cover glass 62, electrode layer 64,polyimide alignment layer 66 and fLC layer 68. Illumination light 38 mayreflect off of backplane 72 (e.g., as image light 22 when thecorresponding pixel P* is turned on). Image light 22 may then passthrough fLC layer 68, polyimide alignment layer 66, electrode layer 64,cover glass 62, and anti-reflective coating 60 before passing towaveguide 26 of FIG. 2 .

fLC layer 68 may have a corresponding birefringence Δn. fLC layer 68 mayhave a thickness 78. Thickness 78 may sometimes be referred to herein ascell gap 78. In general, cell gap 78 may be selected to optimize theoptical efficiency of fLCOS display panel 40 at a particular wavelength.This may be performed by selecting cell gap 78 to be approximately equalto (e.g., within 5% of) λ/(4Δn), where λ is the vacuum wavelength forwhich optical efficiency is optimized and “/” is the division operator.

FIG. 5 is a plot of the optical efficiency of fLC display panel 40 as afunction of cell gap 78. As shown in FIG. 5 , curve 80 plots theefficiency of fLC display panel 40 at blue wavelengths (e.g., at a bluewavelength such as 450 nm). Curve 82 plots the efficiency of fLC displaypanel 40 at green wavelengths (e.g., a green wavelength such as 532 nm).Curve 84 plots the efficiency of fLC display panel 40 at red wavelengths(e.g., a red wavelength such as 633 nm).

In some scenarios, cell gap 78 may be selected to have magnitude G1(e.g., the cell gap corresponding to the intersection of curves 80 and82). This may serve to optimize the efficiency of fLC display panel 40for both blue and green wavelengths. However, the optical performance offLCOS display panel 40 may be further optimized by increasing cell gap78, as shown by arrow 86, to magnitude G2 (e.g., the cell gapcorresponding to the peak of curve 82). By selecting cell gap 78 to havemagnitude G2, the optical efficiency of fLC display panel 40 may beoptimized for green wavelengths. This may serve to increase the overalloptical efficiency of fLCOS display panel 40 in response to illuminationlight 38 relative to scenarios where cell gap 78 has magnitude G1.

In other words, the optical efficiency of fLC display panel 40 may beoptimized when cell gap 78 of FIG. 4 is selected to be equal toλ_(G)/(4Δn), where λ_(G) is a vacuum wavelength such as a greenwavelength, 526 nm, between 520 nm and 530 nm, between 510 nm and 540nm, between 500 nm and 565 nm, less than 565 nm, less than 550 nm, lessthan 540 nm, less than 530 nm, greater than 500 nm, greater than 510 nm,or greater than 520 nm, as examples. Configuring cell gap 78 in this waymay increase the magnitude of cell gap 78 from a magnitude G1 of around620 nm to a magnitude G2 of around 706 nm, as one example. This mayserve to increase the optical efficiency of fLC layer 68 and thus fLCOSdisplay panel 40 by as much as 5% relative to scenarios where cell gap78 has magnitude G1. The example of FIG. 5 is merely illustrative.Curves 80-84 may have other shapes in practice.

In general, the light-emissive portions of light sources 48A-C (FIG. 3 )emit unpolarized illumination light. The unpolarized illumination lightis converted to a single linear polarization (e.g., s-polarized light orp-polarized light) in order to be reflected by fLCOS display panel 40 asimage light 22. However, if care is not taken, converting unpolarizedlight to light of a single linear polarization can prevent as much ashalf of the emitted illumination light from being converted into imagelight 22, thereby limiting the overall optical efficiency of displaymodule 14A. If desired, light sources 48A-C may include polarizationrecycling structures that increase the amount of emitted illuminationlight that is converted to image light 22, thereby maximizing theoptical efficiency of display module 14A.

FIG. 6 is a top view of an illustrative light source 48 havingpolarization recycling structures. Light source 48 of FIG. 6 may be alight source such as light source 48A, light source 48B, or light source48C of FIG. 3 , for example. An arrangement in which light source 48 isan LED light source is described herein as an example. This is merelyillustrative and, in general, light source 48 may be any desired type oflight source.

As shown in FIG. 6 , light source 48 may include reflector and contactlayer 92. Light source 48 may include an LED die such as LED die 90layered over reflector and contact layer 92. Light source 48 may alsoinclude polarization recycling structures such as polarization recyclingstructures 93. In the example of FIG. 6 , polarization recyclingstructures 93 include a reflective polarizer such as reflectivepolarizer 96 overlapping LED die 90 and a polarizer such as polarizer 94overlapping reflective polarizer 96 (e.g., reflective polarizer 96 maybe optically interposed between polarizer 94 and LED die 90).Polarization recycling structures 93 may, for example, be opticallyinterposed between LED die 90 and prism 46 of FIG. 3 .

Polarizer 94 may transmit light of a single linear polarization whileblocking light of other polarizations. An example in which polarizer 94transmits s-polarized light while blocking light of other polarizationsis described herein as an example. This is merely illustrative and, inanother suitable arrangement, polarizer 94 may transmit p-polarizedlight.

As shown in FIG. 6 , when light source 48 is active, LED die 90 may emitunpolarized illumination light, as shown by arrow 98 (e.g., in responseto control signals received from control circuitry 16 over control path42(s) of FIG. 2 ). In scenarios where reflective polarizer 96 isomitted, polarizer 94 serves to pass s-polarized light from theillumination light (e.g., as illumination light 52 that is provided toprism 46 of FIG. 3 ) while blocking other polarizations. This mayprevent as much as half of the emitted unpolarized light from passing toprism 46 and thus fLCOS display panel 40 (FIG. 3 ). Polarizationrecycling structures 93 may serve to recycle polarizations of light thatwould otherwise not be transmitted by polarizer 94 until at least someof the recycled light passes through polarizer 94 as illumination light52, thereby increasing the overall optical efficiency of the displaymodule.

Reflective polarizer 96 may be, for example, a wire grid polarizer(WGP), a reflective polarizer film or coating, a cholesteric liquidcrystal (LC) layer, or other structures that transmit light of a firstpolarization while reflecting light of a second polarization. As shownby arrow 100, reflective polarizer 96 may transmit light of the samepolarization that is transmitted by polarizer 94 (e.g., reflectivepolarizer 96 may transmit s-polarized light). This light may passthrough polarizer 94 as a portion of the illumination light 52 that isprovided to prism 46 of FIG. 3 . At the same time, reflective polarizer96 may reflect light of other polarizations that are not transmitted bypolarizer 94.

For example, as shown by arrow 102, reflective polarizer 96 may reflectp-polarized light from the unpolarized light emitted by LED die 90. Thep-polarized light reflected by reflective polarizer 96 may reflect offof reflector and contact layer 92, as shown by arrow 104. Some of thep-polarized light associated with arrow 102 may be converted tos-polarized light in the process of passing through LED die 90 andreflecting off of reflector and contact layer 92. This s-polarized lightmay be transmitted by reflective polarizer 96 and polarizer 94 as aportion of illumination light 52 (e.g., as shown by arrow 106). At thesame time, the p-polarized light associated with arrow 104 may reflectback to reflector and contact layer 92, as shown by arrow 108. The lightmay continue to reflect between reflective polarizer 96 and reflectorand contact layer 92 (e.g., an infinite number of times), withs-polarized light in the reflected light passing through reflectivepolarizer 96 and polarizer 94 (e.g., as a portion of illumination light52) for each reflection (bounce). Each bounce may contribute mores-polarized light to illumination light 52, thereby increasing the totalamount of the light emitted by light source 48 that passes to prism 46and fLCOS display panel 40 as illumination light 38 (FIG. 3 ). This mayserve to increase the overall optical efficiency of the display modulerelative to scenarios where reflective polarizer 96 is omitted.

In order to further increase the optical efficiency of the displaymodule, polarization recycling structures 93 may include a quarter waveplate. FIG. 7 is a diagram showing how polarization recycling structures93 may include a quarter waveplate (QWP). As shown in FIG. 7 ,polarization recycling structures 93 may include a quarter waveplatesuch as quarter waveplate 120. Quarter waveplate 120 may be opticallyinterposed between reflective polarizer 96 and LED die 90.

As shown by arrow 122 of FIG. 7 , the unpolarized light emitted by LEDdie 90 may pass through quarter waveplate 120 to reflective polarizer96. Reflective polarizer 96 may transmit light of the same polarizationthat is transmitted by polarizer 94 (e.g., reflective polarizer 96 maytransmit s-polarized light). As shown by arrow 124, this s-polarizedlight may pass through polarizer 94 as a portion of illumination light52. At the same time, reflective polarizer 96 may reflect p-polarizedlight back towards quarter wave plate 120, as shown by arrow 126.

Quarter waveplate 120 may convert the p-polarized light reflected byreflective polarizer 96 into right-hand circularly polarized (RHCP)light that is transmitted to reflector and contact layer 92, as shown byarrow 128. The RHCP light transmitted by quarter waveplate 120 mayreflect off of reflector and contact layer 92 as left-hand circularlypolarized (LHCP) light, as shown by arrow 130. Quarter waveplate 120 mayconvert the LHCP light associated with arrow 130 into s-polarized light.As shown by arrow 132, the s-polarized light transmitted by quarterwaveplate 120 may pass through reflective polarizer 96 and polarizer 94to form a portion of illumination light 52. Including quarter waveplate120 in polarization recycling structures 93 may serve to increase theamount of emitted light that is converted into illumination light 52relative to scenarios where waveplate 120 is omitted (e.g., because theLHCP light associated with arrow 130 is converted to s-polarized lightwithout the need for additional reflections between reflective polarizer96 and reflector and contact layer 92). This may serve to increase theoverall optical efficiency of the display module relative to scenarioswhere polarization recycling structures 93 do not include quarterwaveplate 120 (e.g., as shown in FIG. 6 ). If desired, quarter waveplate120 may have a retardation value optimized for each RGB LEDrespectively. For example, for red light source 48A (FIG. 3 ), theretardation value dΔn may be approximately λ_(R)/4, where λ_(R) is thepeak wavelength of red light source 48A. Polarizer 94 may be omittedfrom polarization recycling structures 93 as described herein, ifdesired (e.g., because the reflective polarizer 96 outputs linearlypolarized light).

FIGS. 8 and 9 are plots showing how polarization recycling structures 93may optimize the optical performance of display module 14A. In FIG. 8 ,the horizontal axis plots incident angle (in degrees) and the verticalaxis plots the luminance of the illumination light 52 produced by lightsource 48. Curve 140 of FIG. 8 plots the luminance as a function ofincident angle in scenarios where polarization recycling structures 93are omitted (e.g., scenarios in which only a polarizer such as polarizer94 is used to convert the unpolarized light emitted by LED die 90 intopolarized light for reflection by the fLCOS display panel).

Curve 142 plots the luminance as a function of incident angle for theexample of FIG. 6 in which light source 48 includes polarizationrecycling structures 93 having reflective polarizer 94. Curve 144 plotsthe luminance as a function of incident angle for the example of FIG. 7in which light source 48 includes polarization recycling structures 93having both reflective polarizer 94 and quarter waveplate 120. As shownby curves 142 and 144, polarization recycling structures 93 may increasethe luminance of light source 48 across all incident angles. Includingquarter waveplate 120 may, for example, further increase the luminanceof light source 48.

In FIG. 9 , the horizontal axis plots integrating cone angle and thevertical axis plots the optical efficiency improvement obtained by thedisplay module relative to scenarios where polarization recyclingstructures 93 are omitted (e.g., scenarios in which only a polarizersuch as polarizer 94 is used to convert the unpolarized light emitted byLED die 90 into polarized light for reflection by the fLCOS displaypanel).

Curve 146 of FIG. 9 plots the efficiency improvement for the example ofFIG. 6 in which light source 48 includes polarization recyclingstructures 93 having reflective polarizer 96. Curve 148 plots theefficiency improvement for the example of FIG. 7 in which light source48 includes polarization recycling structures 93 having both reflectivepolarizer 96 and quarter waveplate 120. As shown by curves 148 and 146,polarization recycling structures 93 may increase the efficiency ofdisplay module 14A for all integrating cone angles relative to scenarioswhere polarization recycling structures 93 are omitted. As shown bycurve 148, including quarter waveplate 120 in polarization recyclingstructures 93 may further increase the optical efficiency of the displaymodule 14A, particularly at larger integrating cone angles. The examplesof FIGS. 8 and 9 are merely illustrative. Curves 140-148 may have othershapes in practice.

Polarization recycling structures 93 may be optically interposed betweenLED die 90 and prism 46 (FIG. 3 ) in any desired manner. FIG. 10 is across-sectional side view showing one illustrative example of howpolarization recycling structures 93 may be integrated within lightsource 48. As shown in FIG. 10 , light source 48 may include a substratesuch as patterned sapphire substrate (PSS) 150 layered over LED die 90.Polarization recycling structures 93 may be layered over PSS 150.

In another suitable arrangement, polarization recycling structures 93may be separated from PSS 150 by an air gap. FIG. 11 is across-sectional side view showing how polarization recycling structures93 may be separated from PSS 150 by an air gap in an example wherepolarization recycling structures 93 include a wire grid polarizer.

As shown in FIG. 11 , polarization recycling structures 93 in lightsource 48 may be separated PSS 150 by air gap 168. Polarizationrecycling structures 93 may include a substrate such as glass layer 162(sometimes referred to herein as cover glass layer 162) that isseparated from PSS 150 by air gap 168. In another suitable arrangement,layer 162 may include sapphire or other optically transparent materialsif desired. Polarization recycling structures 93 may include a wire gridpolarizer such as wire grid polarizer 164 (e.g., a wire grid polarizerthat forms reflective polarizer 96 of FIGS. 6 and 7 ) patterned onto thesurface of glass layer 162 facing PSS 150. If desired, the opposingsurface of glass layer 162 may be covered by an optional anti-reflectivelayer (coating) 166. If desired, PSS 150 may include textured surfacefeatures (e.g., surface roughness) 160 on the surface of PSS 150 at airgap 168. Textured surface features 160 may, for example, increase lightextraction efficiency and/or improve emission uniformity through glasslayer 162 for light source 48 relative to scenarios where texturedsurface features 160 are omitted.

The example of FIG. 11 in which the reflective polarizer includes wiregrid polarizer 164 is merely illustrative. In another suitablearrangement, the reflective polarizer may include a reflective polarizerfilm. FIG. 12 is a cross-sectional side view showing how polarizationrecycling structures 93 may be separated from PSS 150 by an air gap inan example where polarization recycling structures 93 include areflective polarizer film.

As shown in FIG. 12 , polarization recycling structures 93 may includereflective polarizer film 170. Reflective polarizer film 170 may beseparated from PSS 150 by air gap 168. Reflective polarizer film 170 mayform reflective polarizer 96 of FIGS. 6 and 7 . An optionalanti-reflective layer (coating) 169 may be layered onto the surface ofreflective polarizer firm 170 facing PSS 150. In examples wherepolarization recycling structures 93 include quarter waveplate 120 (FIG.7 ), quarter waveplate 120 may be layered onto the bottom surface ofreflective polarizer film 170 (e.g., quarter waveplate 120 may beinterposed between reflective polarizer film 170 and anti-reflectivelayer 169 or PSS 150).

Reflective polarizer film 170 may be adhered to glass layer 162 byadhesive layer 172. Adhesive layer 172 may include optically clearadhesive, pressure sensitive adhesive, or other adhesives, as examples.One or both of anti-reflective layers 166 and 169 may be omitted ifdesired. Inclusion of air gap 168 in light source 48 may, for example,allow for a fixed distance to be maintained between the reflectivepolarizer (e.g., wire grid polarizer 164 of FIG. 11 or reflectivepolarizer film 170 of FIG. 12 ) and LED die 90 (e.g., a distance of 200microns or less, 100 microns or less, etc.).

If desired, in scenarios where light source 48 includes air gap 168, LEDdie 90 and polarization recycling structures 93 may be integrated into asingle LED package on a ceramic substrate. FIG. 13 is a cross-sectionalside view showing how LED die 90 and polarization recycling structures93 may be integrated into a single LED package on a ceramic substrate.

As shown in FIG. 13 , light source 48 may include LED chip 180 mountedto a substrate such as ceramic substrate 184. Other materials may beused to form substrate 184 if desired. LED chip 180 may include LED die90 and reflector and contact layer 92 of FIGS. 6, 7, and 10-12 , forexample. PSS 150 may be layered over LED chip 180. In the example ofFIG. 13 , polarization recycling structures 93 include adhesive layer172, reflective polarizer film 170, and anti-reflective layer 169layered onto the bottom surface of glass layer 162. This is merelyillustrative and, in another suitable arrangement, polarizationrecycling structures 93 may include wire grid polarizer 164 of FIG. 11 .Anti-reflective layer 169 may be omitted if desired.

Light source 48 may include spacer and sealant 182 that couples glasslayer 162 to ceramic substrate 184 (e.g., surrounding a lateralperiphery of polarization recycling structures 93 and chip 180). Spacerand sealant 182 may hold glass layer 162 in place over chip 180 suchthat polarization recycling structures 93 are separated from PSS 150 byair gap 168.

Polarization recycling structures 93 of FIGS. 6, 7, and 10-13 may beused to cover a single light source 48. In another suitable arrangement,the same polarization recycling structures 93 may be shared by multiplelight sources 48. FIG. 14 is a cross-sectional side view showing howmultiple light sources 48 may share the same polarization recyclingstructures 93.

As shown in FIG. 14 , multiple light sources 48 (e.g., multiple lightsources 48 that emit light of the same color and that are arranged in anarray) may collectively form a light source set 199 (sometimes referredto herein as light source array 199). The light sources 48 in lightsource set 199 may be arranged in a one-dimensional array pattern or ina two dimensional array pattern, as examples. Each light source 48 inlight source set 199 may produce corresponding illumination light 52(e.g., polarized illumination light to be provided to prism 46 of FIG. 3).

Each light source 48 in light source set 199 may include a correspondingemitter 198 mounted to a common (shared) substrate such as siliconsubstrate 200. Silicon substrate 200 may, for example, be a silicondriver that drives emitters 198 to emit unpolarized illumination light(e.g., based on control signals received from control circuitry 16 overcontrol path(s) 42 of FIG. 2 ). The emitters 198 in light source set 199may collectively form an emitter array 196 for light source set 199.Each emitter 198 in emitter array 196 may include a corresponding LEDdie 90 and reflector and contact layer 92 of FIGS. 6, 7, and 10-12(e.g., a corresponding LED chip 180 of FIG. 13 ), for example.

As shown in FIG. 14 , the same substrate such as sapphire substrate 194may be layered over each of the emitters 198 in light source set 199.Similarly, the same polarization recycling structures 93 may be layeredover each of the emitters 198 in light source set 199. Spacer andsealant 192 may separate polarization recycling structures 93 fromsapphire substrate 194 by air gap 168. In the example of FIG. 14 ,polarization recycling structures 93 include adhesive layer 172,reflective polarizer film 170, and anti-reflective layer 169 layeredonto the bottom surface of glass layer 162. This is merely illustrativeand, in another suitable arrangement, polarization recycling structures93 may include wire grid polarizer 164 of FIG. 11 . Anti-reflectivelayer 169 and/or anti-reflective layer 166 may be omitted if desired.

If desired, light source 48 may include a condenser lens. In thesearrangements, if desired, polarization recycling structures 93 may beintegrated with the condenser lens. FIGS. 15 and 16 are cross-sectionalside views showing how light source 48 may include polarizationrecycling structures 93 integrated with a condenser lens.

As shown in FIG. 15 , light source 48 may include an LED emission area214 mounted to substrate 211. Substrate 211 may, for example, includeceramic substrate 184 of FIG. 13 . LED emission area 214 may include LEDchip 180 and PSS 150 of FIG. 13 , PSS 150, LED die 90, and reflector andcontact layer 92 of FIGS. 6, 7, and 10-12 , etc. Light source 48 mayinclude a lens such as condenser lens 210 overlapping LED emission area214. Polarization recycling structures 93 may be layered onto the bottom(e.g., planar) surface of condenser lens 210. Spacer and sealant 182 mayseparate polarization recycling structures 93 from LED emission area 214by air gap 168. Condenser lens 210 may help to focus and/or redirect theillumination light 52 produced by light source 48.

The example of FIG. 15 in which spacer and sealant 182 polarizationrecycling structures 93 cover an entirety of the bottom surface ofcondenser lens 210 is merely illustrative. In another suitablearrangement, polarization recycling structures 93 may cover only theportion of condenser lens 210 overlapping LED emission area 214, asshown in FIG. 16 . In this example, spacer and sealant 182 may separatethe bottom surface of condenser lens 210 from substrate 211 such thatpolarization recycling structures 93 are separated from LED emissionarea 214 by air gap 168.

In the example of FIGS. 15 and 16 , polarization recycling structures 93include adhesive layer 172 and reflective polarizer film 170 layeredonto the bottom surface of condenser lens 210. This is merelyillustrative and, if desired, polarization recycling structures 93 mayinclude wire grid polarizer 164 of FIG. 11 or any other desiredstructures. Quarter waveplate 120 of FIG. 7 may be layered onto thebottom surface of reflective polarizer film 170 or may be otherwiseoptically interposed between reflective polarizer film and the LEDemission area in any of the examples of FIGS. 11-16 , if desired. Lightsource 48 may include other structures for producing polarizedillumination light 52 if desired.

Polarizing illumination light 52 prior to passing illumination light 52to prism 46 of FIG. 17 may serve to optimize the optical performance ofthe display module. For example, as shown in the top-down view of FIG.17 , prism 46 in illumination optics 36 may include an X-plate formedfrom a first partial reflector 220 that intersects with a second partialreflector 222. First partial reflector 220 may include coating 224.Second partial reflector 222 may include coating 226. Coatings 224 and226 may sometimes be referred to herein as material interfaces and mayinclude laminated interference films, diffractive elements that serve asa beam combiner, or other types of coatings or material interfaces.Prism 46 may include the X-plate formed from partial reflectors 220 and222 with or without optical wedges between the partial reflectors.

Coatings 224 and 226 may be wavelength-selective filters that configurepartial reflectors 220 and 222 to reflect illumination light ofcorresponding wavelengths while transmitting light of other wavelengths.For example, coating 226 may configure partial reflector 222 to reflectillumination light of the wavelengths produced by light source 48A(e.g., red illumination light 52A) while transmitting illumination lightof the wavelengths produced by light sources 48B and 48C. Coating 224may configure partial reflector 220 to reflect illumination light of thewavelengths produced by light source 48C (e.g., blue illumination light52C) while transmitting illumination light of the wavelengths producedby light sources 48A and 48B. The illumination light transmitted bylight source 48B (e.g., green illumination light 52B) may be transmittedby partial reflectors 220 and 222 without being reflected. In this way,the X-plate (e.g., prism 46) may serve as a beam combiner that combinesillumination light 52A, 52B, and 52C to produce illumination light 38.

Illumination light 52A-C may be polarized illumination light (e.g.,polarized illumination light as produced by polarization recyclingstructures 93 of FIGS. 6, 7, and 10-16 ). The illumination light 38produced by prism 46 will therefore have the same polarization asillumination light 52A-C. Polarizing illumination light 52A-C prior tothe illumination light passing through prism 46 may serve to optimizethe spectral performance of illumination light 38, for example.

FIG. 18 is a plot showing how polarizing illumination light 52A-C priorto the illumination light passing through prism 46 may serve to optimizethe spectral performance of illumination light 38. The horizontal axisof FIG. 18 plots wavelength (e.g., in nm) and the vertical axis of FIG.18 plots the amount of reflection performed by prism 46 (e.g., where 0%reflection corresponds to an entirety of the illumination light beingtransmitted by prism 46 and 100% corresponds to an entirety of theillumination light being reflected by prism 46).

Curve 230 of FIG. 8 plots the reflection, by prism 46, of theillumination light 52C emitted by light source 48C (e.g., blueillumination light) in scenarios where illumination light 52C isunpolarized. Curve 232 plots the reflection, by prism 46, of theillumination light 52A emitted by light source 48A (e.g., redillumination light) in scenarios where illumination light 52A isunpolarized. As shown by curve 230, partial reflector 220 and coating224 (FIG. 17 ) may exhibit a relatively shallow roll-off in reflectingunpolarized blue light as wavelength increases. Similarly, as shown bycurve 232, partial reflector 222 and coating 226 may exhibit arelatively shallow roll-off in reflecting unpolarized red light aswavelength decreases.

Curve 234 plots the reflection, by prism 46, of the illumination light52C emitted by light source 48C (e.g., blue illumination light) inscenarios where illumination light 52C is polarized (e.g., bypolarization recycling structures 93 of FIGS. 6, 7, and 10-16 ). Curve236 plots the reflection, by prism 46, of the illumination light 52Aemitted by light source 48A (e.g., red illumination light) in scenarioswhere illumination light 52A is polarized (e.g., by polarizationrecycling structures 93 of FIGS. 6, 7, and 10-16 ).

As shown by curve 234 and arrow 238, providing polarized blueillumination light to prism 46 may cause partial reflector 220 andcoating 224 to exhibit a steeper roll-off in reflecting blue light aswavelength increases than in scenarios where unpolarized blue light isprovided to prism 46. Similarly, as shown by curve 236 and arrow 240,providing polarized red illumination light to prism 46 may cause partialreflector 222 and coating 226 to exhibit a steeper roll-off inreflecting red light as wavelength decreases than in scenarios whereunpolarized red light is provided to prism 46. This may serve tooptimize the spectral response of the illumination light 38 output byprism 46, for example. The example of FIG. 18 is merely illustrative.Curves 230-236 may have other shapes in practice. In general, prism 46may combine illumination light 52 of any desired wavelengths to produceillumination light 38 that is provided to fLCOS display panel 40 of FIG.3 .

In general, the efficiency of the LEDs in light sources 48 may depend onthe current density used to drive the LEDs. In addition, different colorLEDs exhibit peak LED efficiency at different current densities. Inpractice, green LEDs such as an LED in light source 48B may reach peakLED efficiency at a lower current density than red LEDs (e.g., in lightsource 48A) and/or blue LEDs (e.g., in light source 48C). In order toreduce the overall power consumption of display module 14A, light source48B may therefore be driven with a lower current density than lightsources 48A and/or 48C.

The light sources 48A-C in illumination optics 36 may be driven using acorresponding illumination sequence. The illumination sequence mayspecify the order in which each light source 48 is activated to produceillumination light 38. In some scenarios, the illumination scheme is anRGBRGB illumination scheme. However, if care is not taken, driving lightsources 48 using an RGBRGB illumination scheme while reducing thecurrent density used to drive light source 48B may cause illuminationlight 38 to exhibit less overall brightness at green wavelengths. Thismay lead to an unsightly color and brightness imbalance in the imagesproduced at eye box 24 (FIG. 2 ). In order to mitigate these issueswhile driving light source 48B with a reduced current density, lightsources 48A-C may be driven using a green-heavy illumination sequence.

FIG. 19 is a timing diagram of illustrative illumination sequences thatmay be used to drive light sources 48A-C. As shown in FIG. 19 , anRGBRGB illumination sequence 250 may be used to drive light sources48A-C in some scenarios. RGBRGB illumination sequence 250 may involvethe sequential activation of only one of light sources 48A-C at anygiven time.

Under RGBRGB illumination sequence 250, for a given image frame, redlight source 48A may be active for a first time period (slot) 252,during which red light source 48A emits red (R) illumination light 52Aof FIGS. 3 and 17 (e.g., illumination light as polarized usingpolarization recycling structures 93 of FIGS. 6, 7, and 10-16 ). Greenlight source 48B and blue light source 48C may be inactive during thefirst time period 252 (e.g., green light source 48B and blue lightsource 48C may not emit any illumination light during the first timeperiod 252). Green light source 48B may be active for a subsequentsecond time period 252, during which green light source 48B emits green(G) illumination light 52B. Red light source 48A and blue light source48C may be inactive during the second time period 252 (e.g., red lightsource 48A and blue light source 48C may not emit any illumination lightduring the second time period 252). Blue light source 48C may be activeduring a subsequent third time period 252, during which blue lightsource 48C emits blue (B) illumination light 52C. Red light source 48Aand green light source 48B may be inactive during the third time period252 (e.g., red light source 48A and green light source 48B may not emitany illumination light during the third time period 252). Red lightsource 48A may be active during a subsequent fourth time period 252,green light source 48B may be active during a subsequent fifth timeperiod 252, and blue light source 48C may be active during a subsequentsixth time period 252 (e.g., each light source may be active during twotime periods 252 for a given image frame to be displayed by displaymodule 14A).

In order to minimize power consumption by illumination optics 36, greenlight source 48B may be driven using lower current density than thegreen light source would have otherwise been driven under a differentillumination sequence for a given field (e.g., while recovering similarvisual performance). In order to recover the same overall brightness atgreen wavelengths as would otherwise be obtained if a higher currentdensity were used to drive green light source 48B, light sources 48A-Cmay be driven using green-heavy illumination sequence 254 of FIG. 19 .

Green-heavy illumination sequence 254 may include three time periods(slots) 256 that are used to produce illumination light 38 for a givenimage frame (e.g., a first time period 256-1, a subsequent second timeperiod 256-2, and a subsequent third time period 256-3). Each timeperiod 256 may correspond to an image subframe (field) that is displayedusing fLCOS display panel 40. Both red light source 48A and green lightsource 48B may be active for first time period 256-1. During first timeperiod 256-1, red light source 48A may emit red (R) illumination light52A and green light source 48B may emit green (G) illumination light52B. Prism 46 (FIGS. 3 and 17 ) may combine illumination light 52A and52B to produce illumination light 38. Blue light source 48C may beinactive during first time period 256-1.

Green light source 48B may be active for second time period 256-2.During second time period 256-2, green light source 48B may emit greenillumination light 52B. Prism 46 (FIGS. 3 and 17 ) may produceillumination light 38 based on the green illumination light 52B. Redlight source 48A and blue light source 48C may be inactive during secondtime period 256-2.

Both blue light source 48C and green light source 48B may be active forthird time period 256-3. During third time period 256-3, blue lightsource 48C may emit blue (B) illumination light 52C and green lightsource 48B may emit green illumination light 52B. Prism 46 (FIGS. 3 and17 ) may combine illumination light 52C and 52B to produce illuminationlight 38. Red light source 48A may be inactive during third time period256-3.

In other words, green light source 48B may be active during each of thetime periods 256 used to display a corresponding image frame (e.g.,green light source 48B may contribute to the blue and red portions ofthe illumination sequence). By contributing green illumination light 52Bto illumination light 38 in each time period 256 (e.g., by increasingthe total on time for green light source 40B per image frame), the totalillumination time for the green light source may be greater than inscenarios where RGBRGB illumination sequence 250 is used. This may allowgreen light source 48B to be driven with lower current density withoutsignificantly sacrificing optical performance, thereby minimizing powerconsumption in display module 14A.

The example of FIG. 19 is merely illustrative. If desired, othergreen-heavy illumination sequences having any desired number of periods256 may be used (e.g., illumination sequences where green light source48B is active during a greater number of time periods 256 per frame thanred light source 48A and blue light source 48C). If desired, red lightsource 48A and/or blue light source 48C may be active during second timeperiod 256-2 (e.g., where red light source 48A is driven using lesscurrent density than during time period 256-1 and where blue lightsource 48C is driven using less current density than during time period256-3). Light sources 48A-C may emit illumination light of anyrespective colors, in general.

FIG. 20 is a flow chart of illustrative steps that may be performed bysystem 10 to display images using a green-heavy illumination sequencesuch as green-heavy illumination sequence 254 of FIG. 19 .

At step 260, control circuitry 16 (FIG. 2 ) may process image data to bedisplayed at eye box 24. The image data may include a stream of imageframes. Control circuitry 16 may determine whether a trigger conditionhas been met before beginning to display images using the green-heavyillumination sequence.

If desired, control circuitry 16 may determine whether the triggercondition has been met based on the content of the image data to bedisplayed. For example, control circuitry 16 may determine that thetrigger condition has been met when one or more image frames to bedisplayed exhibit a saturation level that exceeds a threshold saturationlevel (e.g., a green saturation level that exceeds a threshold greensaturation level). If desired, the green-heavy illumination sequence maybe disregarded in favor of another illumination sequence (e.g., RGBRGBillumination sequence 250 of FIG. 19 ) in scenarios where use of agreen-heavy illumination sequence is unlikely to result in animprovement in power consumption and/or optical performance. This ismerely illustrative and, in general, any desired trigger condition maybe used (e.g., a command to begin using the green-heavy illuminationsequence issued by a software call on system 10, a command to beginusing the green-heavy illumination sequence as identified by user inputprovided to system 10, etc.). In some examples, the above triggercondition(s) may be used when the optical system is free of chromaticaberration. In one suitable arrangement that is sometimes describedherein as an example (e.g., in scenarios where chromatic aberration ispresent), the trigger condition may be an ambient light level identifiedby ambient light sensor data collected by one or more ambient lightsensors in system 10. If desired, different green light doping ratiosmay be used (e.g., in the green-heavy illumination sequence) based onthe current measured ambient light level (e.g., control circuitry 16 mayadjust the relative amount of green illumination in each of the timeperiods of the illumination sequence based on the ambient light sensordata such that different relative amounts are used when differentambient light levels are detected). This may help to ensure thatchromatic aberration artifacts remain invisible to the eye, for example

When the trigger condition has been met, processing may proceed to step264, as shown by arrow 262. At step 264, control circuitry 16 maycontrol light sources 48A-C to generate illumination light 38 using thegreen-heavy illumination sequence. Control circuitry 16 may, forexample, provide driving signals to light sources 48A-C over controlpath(s) 42 (FIG. 2 ) (e.g., driving signals with a corresponding currentdensity) that selectively activate light sources 48A-C in accordancewith the green-heavy illumination sequence (e.g., green-heavyillumination sequence 254 of FIG. 19 ) for each image frame to bedisplayed. Control circuitry 16 may drive green light source 48B withlower current density than for display of the same image data usingRGBRGB illumination sequence 250, minimizing power consumption in system10 by meeting the peak efficiency of the green LED in green light source48B.

If desired, step 266 may be performed concurrently with step 264. Atstep 266, control circuitry 16 may provide image data to fLCOS displaypanel 40 (FIG. 3 ). The image data may include image frame(s) (e.g., asprocessed at step 260). Each image frame may be used to control eachpixel P* in fLCOS display panel 40 to modulate illumination light 38(e.g., illumination light as generated in accordance with thegreen-heavy illumination scheme) to produce corresponding image light22.

Each image frame may be divided into sub-frames or fields to bedisplayed during each time period 256 of the green-heavy illuminationsequence (FIG. 19 ). For example, for a given image frame, a firstsub-frame (field) of the image frame may be driven onto fLCOS displaypanel 40 during time period 256-1 of FIG. 19 (e.g., for producing afirst sub-frame in image light 22 using the polarized red and greenillumination light produced during time period 256-1), a secondsub-frame (field) of the image frame may be driven onto fLCOS displaypanel 40 during time period 256-2 (e.g., for producing a secondsub-frame in image light 22 using the polarized green illumination lightproduced during time period 256-2), and a third sub-frame (field) of theimage frame may be driven onto fLCOS display panel 40 during time period256-3 (e.g., for producing a third sub-frame in image light 22 using thepolarized green and blue illumination light produced during time period256-3). If desired, control circuitry 16 may perform chromaticaberration compensation operations when driving fLCOS display panel 40with the image data (optional step 268).

At step 270, optical system 14B (FIG. 2 ) may direct the image light 22produced by display module 14A towards eye box 24. Processing maysubsequently loop back to step 260, as shown by arrow 272, as additionalimage frames are processed for display at the eye box. Control circuitry16 may cycle through these steps rapidly enough so that each of thedifferent-colored sub-frames appears at eye box 24 as a series ofmulti-color image frames to the user at eye box 24 (e.g., image frameshaving a corresponding color gamut and that appears visually similar tohow the image frames appear to the user in scenarios where green lightsource 48B is driven with higher current density using an RGBRGBillumination sequence). In this way, power consumption in display module14A may be minimized without significantly reducing image quality at eyebox 24.

FIG. 21 is a flow chart of illustrative steps that may be performed bycontrol circuitry 16 in driving light sources 48A-C using thegreen-heavy illumination sequence (e.g., green-heavy illuminationsequence 254 of FIG. 19 ). The steps of FIG. 21 may, for example, beperformed while processing step 264 of FIG. 20 (e.g., for a given imageframe to be displayed at the eye box).

At step 280 of FIG. 21 , control circuitry 16 may concurrently activate(turn on) red light source 48A and green light source 48B to produce redillumination light 52A and green illumination light 52B (e.g., duringtime period 256-1 of FIG. 19 ). This may produce a correspondingsub-frame (field) of the image frame having a color given by thecombination of red illumination light 52A and green illumination light52B. Blue light source 48C may be inactive (turned off).

At step 282, control circuitry 16 may activate (turn on) green lightsource 48B to produce green illumination light 52B (e.g., during timeperiod 256-2 of FIG. 19 ). This may produce a corresponding sub-frame(field) of the image frame having a green color given by greenillumination light 52B. Red light source 48A and blue light source 48Cmay be inactive (turned off).

At step 284, control circuitry 16 may concurrently activate (turn on)blue light source 48C and green light source 48B to produce blueillumination light 52C and green illumination light 52B (e.g., duringtime period 256-3 of FIG. 19 ). This may produce a correspondingsub-frame (field) of the image frame having a color given by thecombination of blue illumination light 52C and green illumination light52B. Red light source 48A may be inactive (turned off). Processing maysubsequently loop back to step 280, as shown by arrow 285, as additionalimage frames are displayed. The steps of FIG. 21 are merely illustrativeand may, in general, be adapted to the particular green-heavyillumination sequence that is used to produce illumination light 38.

FIG. 22 is a flow chart of illustrative steps that may be performed bycontrol circuitry 16 in performing chromatic aberration compensationoperations while driving fLCOS display panel 40 with the image data(e.g., while producing image light 22 using green-heavy illuminationsequence 254 of FIG. 19 ). The steps of FIG. 22 may, for example, beperformed while processing step 268 of FIG. 20 (e.g., for a given imageframe to be displayed at the eye box). The steps of FIG. 22 may beperformed to compensate for chromatic aberrations introduced into imagelight 22 by collimating lens 34 and/or any other desired opticalcomponents in display module 14A and/or optical system 14B (FIG. 2 ).

At step 290, control circuitry 16 may identify an image frame to bedriven onto fLCOS display panel 40 for producing image light 22 inresponse to illumination light 38.

At step 292, control circuitry 16 may decompose the image frame into ared (R) LED channel image (sub-frame), a blue (B) LED channel image(sub-frame), and a green (G) LED channel image (sub-frame), for example.

At step 294, control circuitry 16 may pre-compensate the red, blue, andgreen LED channel images for chromatic aberration that will beintroduced into image light 22 by the optical components of system 10(e.g., control circuitry 16 may generate chromatic aberrationpre-compensated red, blue, and green channel images). The amount ofpre-compensation that needs to be introduced to each channel image tocompensate for chromatic aberration may, for example, be determinedduring the design, manufacture, assembly, and/or testing of system 10(e.g., in a manufacturing, testing, or calibration system). Thepre-compensation may be performed, for example, by shifting the relativepixel position of portions of the image frame that will be subject tochromatic aberrations by different amounts across each of the colorchannels / fields.

At step 296, control circuitry 16 may perform green redistributionoperations. For example, control circuitry 16 may first modify the redillumination light from light source 48A to a combination of red andgreen light from light sources 48A and 48B, without changing thecorresponding image data used to drive fLCOS display panel 50 (sometimesreferred to herein as the fLCOS display panel signal). Control circuitry16 may then modify the blue illumination light from light source 48C toa combination of blue and green light from light sources 48B and 48C,without changing the corresponding fLCOS display panel signal. The redand blue illumination light may be modified to include 1-10% greenillumination, between 2-8% green illumination, between 5-20% greenillumination, around 5% green illumination, or any other desired amountof green illumination (sometimes referred to herein as the green lightdoping ratio). Control circuitry 16 may then modify the image data usedto drive fLCOS display panel 50 for the green channel, by subtracting,from the image data for the green channel, image data corresponding tothe amount of green illumination that was added into the red channel(e.g., in modifying the red illumination light as described above) andthe amount of green illumination that was added into the blue channel(e.g., in modifying the blue illumination light as described above).Next, any negative signal values in the modified signal may be changedto zero (e.g., a black level) and excessive green illumination values(e.g., green illumination values that exceed a threshold value) may bechanged to the maximum brightness of the field (e.g., as determined bythe corresponding green light doping ratio).

At step 298, control circuitry 16 may drive fLCOS display panel 40 usingcolor channel images (image data) associated with the green-heavyillumination sequence. For example, control circuitry 16 may drive fLCOSdisplay panel 40 using an (R+G) channel image for the combination of redand green illumination light (e.g., during time period 256-1 of FIG. 19), then using a green (G) channel image as modified during step 296(e.g., during time period 256-2 of FIG. 19 ), then using a (B+G) channelimage for the combination of blue and green light (e.g., during timeperiod 256-3 of FIG. 19 ). The corresponding image light 22 produced byfLCOS display panel 40 may be pre-compensated for chromatic aberrationsby the optical components along the remainder of the optical pathbetween display module 14A and eye box 24 (FIG. 2 ). After passing toeye box 24, the chromatic aberrations introduced by these opticalcomponents may cancel out the pre-compensation in the image light,thereby providing the eye box with images that are free from chromaticaberrations. Processing may subsequently loop back to step 290, as shownby arrow 300, as additional image frames are displayed.

In this way, power consumption may be minimized in display module 14Awithout significantly sacrificing image quality. The green-heavyillumination sequence need not be limited to fLCOS display systems andmay, in general, be used to produce image light 22 in scenarios wheredisplay module 14A includes a DMD display panel, an emissive displaypanel, etc.

Because green light source 48B is turned on more frequently under thegreen-heavy illumination sequence, the green-heavy illumination sequencemay serve to shrink the overall color gamut of display module 14A. FIG.23 is a CIE1931 color space plot showing how the green-heavyillumination sequence may serve to shrink the overall color gamut ofdisplay module 14A. As shown in FIG. 23 , display module 14A may displayimages using a relatively large color gamut 312 (e.g., within overallcolor space 310) in scenarios where a green-heavy illumination sequenceis not used to produce illumination light 38. The green-heavyillumination sequence may serve to reduce the color gamut of displaymodule 14A to color gamut 314, as shown by arrows 316. Reducing thecolor gamut of display module 14A to color gamut 314 may serve to reducethe power consumption of display module 14A relative to scenarios wherean RGBRGB illumination sequence is used, for example. The example ofFIG. 23 is merely illustrative. In general, color space 310, color gamut312, and color gamut 314 may have other shapes.

In practice, it may be desirable to be able to increase both the fieldof view of and the resolution of the images in image light 22 providedto eye box 24. In one suitable arrangement that is described herein asan example, the effective resolution of images provided to eye box 24may be increased by performing pixel shifting operations in display 14.

FIG. 24 is a top-down view showing how display 14 may perform spatialpixel shifting operations to maximize the effective resolution of imagesprovided to eye box 24. As shown in FIG. 24 , display 14 may include atwisted nematic (TN) cell 320 and a birefringent crystal 322 opticallyinterposed between display module 14A (FIG. 2 ) and input coupler 28 onwaveguide 26. Birefringent crystal 322 may be optically interposedbetween TN cell 320 and input coupler 28. If desired, TN cell 320 and/orbirefringent crystal 322 may be formed within display module 14A of FIG.2 (e.g., collimating lens 34 of FIG. 2 may be optically interposedbetween birefringent crystal 322 and input coupler 28).

TN cell 320 may receive image light 22 from fLCOS panel 40 (FIG. 3 ).Image light 22 may be (linearly) polarized light such as p-polarizedlight or s-polarized light. An arrangement in which image light 22 isincident upon TN cell 320 as p-polarized light is described herein as anexample.

TN cell 320 may receive control signals from control circuitry 16 (FIG.2 ) over control path 334. The control signals may toggle TN cell 320between first and second states. In the first state, TN cell 320 maytransmit image light 22 without changing the polarization of image light22. TN cell 320 may thereby transmit p-polarized image light 22 tobirefringent crystal 322 in the first state, as shown by arrow 324. Inthe second state, TN cell 320 may change the polarization of image light22 to a different linear polarization. For example, in the second state,TN cell 320 may convert the p-polarized image light 22 received fromfLCOS display panel 40 into s-polarized image light 22 and may transmitthe s-polarized image light 22 to birefringent crystal 322, as shown byarrow 324.

Birefringent crystal 322 (sometimes referred to herein as birefringentbeam displacer 322) may be formed from a birefringent material such ascalcite and may have a length (thickness) 332 (e.g., in the direction ofthe optical path). Birefringent crystal 322 may be a uniaxialbirefringent crystal or a biaxial birefringent crystal, as examples.Birefringent crystal 322 may receive p-polarized image light 22 ors-polarized image light 22 from TN cell 320 (e.g., depending on thecurrent state of TN cell 320).

Birefringent crystal 322 may spatially separate incident image light 22based on the polarization of the image. For example, birefringentcrystal 322 may output incident s-polarized image light 22 within afirst beam, as shown by arrow 326, and may output incident p-polarizedimage light 22 within a second beam, as shown by arrow 328. Upon exitingbirefringent crystal 322, the second beam (e.g., the p-polarized imagelight 22) may be separated from the first beam (e.g., the s-polarizedimage light 22) by displacement 330. The magnitude of displacement 330may be directly proportional to the length 322 of birefringent crystal322, for example.

The p-polarized image light 22 may be spatially offset from thes-polarized image light 22 upon in-coupling to waveguide 26 by inputcoupler 28 (e.g., by displacement 330). The images conveyed by thes-polarized image light 22 may therefore be spatially offset (e.g., bydisplacement 330) from the images conveyed by the p-polarized imagelight 22 at eye box 24. Control circuitry 16 may rapidly toggle TN cellbetween the first and second states to alternate between providing inputcoupler 28 with p-polarized image light 22 and s-polarized image light22. Length 332 and thus displacement 330 may be selected so that, whenthe state of TN cell 320 is toggled more rapidly than the response rateof the human eye (e.g., 24 Hz or faster, 60 Hz or faster, 120 Hz orfaster, 240 Hz or faster, etc.), the resulting images provided at eyebox 24 exhibit an effective resolution that is greater than theresolution of that would otherwise be conveyed to eye box 24 in theabsence of TN cell 320 and birefringent crystal 322. TN cell 320 andbirefringent crystal 322 of FIG. 24 may sometimes be referred tocollectively herein as spatial pixel shifting structures 325.

The example of FIG. 24 in which display 14 performs spatial pixelshifting operations is merely illustrative. In another suitablearrangement, display 14 may perform angular pixel shifting operations tomaximize the effective resolution of images provided to eye box 24.

FIG. 25 is a top-down view showing how display 14 may perform angularpixel shifting operations to maximize the effective resolution of imagesprovided to eye box 24. As shown in FIG. 25 , birefringent crystal 322of FIG. 24 may be replaced by quarter waveplate 340 and geometric phasegrating (GPG) 342. Quarter waveplate 340 may be optically interposedbetween TN cell 320 and GPG 342. GPG 342 may be optically interposedbetween quarter waveplate 340 and input coupler 28.

Collimating lens 34 (FIG. 2 ) may be optically interposed between GPG342 and input coupler 28, may be optically interposed between quarterwaveplate 340 and GPG 342, may be optically interposed between quarterwaveplate 340 and TN cell 320, or may be optically interposed betweenfLCOS display panel 40 and TN cell 320. An arrangement in whichcollimating lens 34 is optically interposed between quarter waveplate340 and GPG 342 is described herein as an example. In this example, thecollimating lens may serve to focus the pupil of image light 22 onto GPG342 (e.g., GPG 342 may be located external to display module 14A and ator adjacent input coupler 28 and the entrance pupil of waveguide 26),whereas quarter waveplate 340 and TN cell 320 are located within displaymodule 14A.

Quarter waveplate 340 may convert p-polarized image light 22 (e.g., asprovided by TN cell 320 when TN cell 320 is in the first state) intoRHCP light that is provided to GPG 342, as shown by arrow 352. Quarterwaveplate 340 may convert s-polarized image light 22 (e.g., as providedby TN cell 320 when TN cell 320 is in the second state) into LHCP lightthat is provided to GPG 342, as shown by arrow 352.

GPG 342 may diffract incident image light 22 received from quarterwaveplate 340 onto a corresponding output angle θ (e.g., measuredrelative to the optical axis or the Y-axis as shown in FIG. 25 ). GPG342 may have different diffraction orders that diffract incident imagelight 22 in different directions based on the polarization of theincident image light. For example, GPG 342 may have a first diffractionorder (e.g., a +1 diffraction order) that diffracts incident LHCP imagelight 22 onto output angle θ1, as shown by arrow 356. GPG 342 may alsohave a second diffraction order (e.g., a -1 diffraction order) thatdiffracts incident RHCP image light 22 onto output angle -θ2, as shownby arrow 352. Output angle -θ2 may be equal and opposite output angle θ1or may be any other desired output angle. The output angles of arrows354 and 356 may both be oriented on the same side of the optical axis ifdesired.

In one suitable arrangement that is sometimes described herein as anexample, GPG 342 may include a substrate 344 and an alignment layer 346layered onto substrate 344. GPG 342 may include multiple liquid crystal(LC) layers 348 (e.g., a first LC layer 348-1, a second LC layer 348-2,and a third LC layer 348-3) layered onto alignment layer 346. Alignmentlayer 346 may serve to align the LC molecules in LC layers 348 atsubstrate 344 (e.g., with a corresponding grating period). Each LC layer348 may have a corresponding twist angle φ (e.g., LC layer 348-1 mayhave a first twist angle φ₁, LC layer 348-2 may have a second twistangle φ₂ oriented opposite twist angle φ₁, and LC layer 348-3 may have athird twist angle φ₃ oriented opposite twist angle φ₁).

In this way, the LHCP image light 22 may be angularly offset from theRHCP image light 22 upon in-coupling to waveguide 26 by input coupler 28(e.g., by an angular displacement having a magnitude equal to|θ1|+|θ2|). The images conveyed by the LHCP image light 22 may thereforebe angularly offset from the images conveyed by the RHCP image light 22at eye box 24. Control circuitry 16 may rapidly toggle TN cell betweenthe first and second states to alternate between providing input coupler28 GPG 342 and thus input coupler 28 with LHCP image light 22 and RHCPimage light 22. GPG 342 may be configured to output image light 22 atangles θ1 and θ2 that are selected so that, when the state of TN cell320 is toggled more rapidly than the response rate of the human eye, theresulting images provided at eye box 24 exhibit an effective resolutionthat is greater than the resolution of the images that would otherwisebe conveyed to eye box 24 in the absence of TN cell 320, quarterwaveplate 340, and GPG 342. TN cell 320, quarter waveplate 340, and GPG342 of FIG. 25 may sometimes be referred to collectively herein asangular pixel shifting structures 353. Spatial pixel shifting structures325 of FIG. 24 and angular pixel shifting structures 353 may sometimesbe referred to collectively herein as pixel shifting structures fordisplay 14.

FIG. 26 is a front view showing how the pixel shifting structures indisplay 14 may provide image light 22 with an increased effectiveresolution at eye box 24 (e.g., as viewed at eye box 24 in the +Ydirection of FIG. 2 ). In the example of FIG. 26 , four pixels of imagelight 22 are shown for the sake of clarity. In general, image light 22and the display module may include any desired number of pixels.

As shown in FIG. 26 , image light 22 may include pixels P1, P2, P3, andP4 when TN cell 320 of FIGS. 24 and 25 is in the first state (e.g., whenTN cell 320 outputs p-polarized light). When TN cell 320 is in thesecond state (e.g., when TN cell 320 outputs s-polarized light), pixelsP1, P2, P3, and P4 may be displaced by displacement 360, as shown byrespective pixels P1′, P2′, P3′, and P4′. Displacement 360 may, forexample, be a two-dimensional displacement that includes offset 364parallel to the Z-axis and/or offset 362 parallel to the X-axis.Displacement 360 may be produced by a spatial displacement such asdisplacement 330 of FIG. 24 (e.g., in scenarios where the pixel shiftingstructures include spatial pixel shifting structures 325) or by anangular displacement such as an angular displacement having a magnitudeequal to |θ1|+|θ2| of FIG. 25 (e.g., in scenarios where the pixelshifting structures include angular pixel shifting structures 353).

Pixels P1, P2, P3, and P4 may exhibit a first pixel pitch and pixelsP1′, P2′, P3′, and P4′ may also exhibit the first pixel pitch. However,the combination of pixels P1, P2, P3, and P4 with pixels P1′, P2′, P3′,and P4′ may exhibit a second pixel pitch that is less than (e.g., half)the first pixel pitch. By rapidly toggling between the first and secondstates of TN cell 320, image light 22 may effectively include each ofpixels P1, P2, P3, P4, P1′, P2′, P3′, and P4′ (e.g., as perceived by auser at eye box 24) and thus the second pixel pitch, rather than onlypixels P1, P2, P3, and P4 and the first pixel pitch (e.g., in scenarioswhere pixel shifting structures are omitted from display 14). This mayserve to increase the effective resolution of image light 22 relative toscenarios where the pixel shifting structures are omitted (e.g., totwice the resolution that image light 22 would otherwise have in theabsence of the pixel shifting structures), without requiring an increasein size or processing resources for display module 14A.

Control circuitry 16 (FIG. 2 ) may drive image data onto fLCOS displaypanel 40 using fLCOS drive voltage waveforms (e.g., based on controlsignals provided to fLCOS display panel 40 over control path(s) 44 ofFIG. 2 ). FIG. 27 is a timing diagram of two illustrative fLCOS drivevoltage waveforms that may be used to drive fLCOS display panel 40.

As shown in FIG. 27 , fLCOS drive voltage waveform (curve) 370 plots thefLCOS drive voltage as a function of time for producing image light 22with a gray level of zero. fLCOS drive voltage waveform (curve) 372plots the fLCOS drive voltage as a function of time for producing imagelight 22 with a gray level of 128 (e.g., in a 256-bit field). The fLCOSdrive voltage may vary between a first drive voltage V_(OFF) (e.g., anegative voltage level) and a second drive voltage V_(ON) (e.g., apositive voltage level).

Waveforms 370 and 372 may be at first drive voltage V_(OFF) prior totime T0. At time T0, waveform 370 may begin to increase to a peak atsecond drive voltage V_(ON). Waveform 370 may return to first drivevoltage V_(OFF) at time T1. The time period between times T0 and T1 maysometimes be referred to herein as dark gap 374. Dark gap 374 may beused to reset fLCOS display panel 40, for example.

The time period between times T1 and T3 may form a duty period 380during which at least one light source 48 (e.g., red light source 48A ofFIG. 3 ) may be turned on to provide illumination light 38 to fLCOSdisplay panel 40. Because waveform 370 is at first drive voltage V_(OFF)during duty period 380, the fLCOS display panel may not produce imagelight during duty period 380 when driven using waveform 370. The timeperiod between times T0 and T3 may sometimes be referred to as fieldperiod 376. Field period 376 may be associated with the illumination offLCOS display panel 40 by a corresponding field of illumination light(e.g., illumination light of a particular color) and may include thereset time (e.g., a portion of dark gap 374) required to reset the fLCOSdisplay panel to begin reflecting the field of illumination light asimage light 22.

At time T3, waveform 370 may to increase to a peak at second drivevoltage V_(ON). Waveform 370 may return to first drive voltage V_(OFF)at time T4. The time period between times T3 and T4 may sometimes bereferred to herein as the dark gap 382. The time period between time T3and the time when waveform 370 reaches second drive voltage V_(ON) maysometimes be referred to herein as reset time T_RESET. Reset timeT_RESET may allow time for fLCOS display panel 40 to reset for the nextfield of the image. The time period between the time when waveform 370reaches second drive voltage V_(ON) and time T4 may sometimes bereferred to herein as off time T_OFF. The duration of dark gap 374(e.g., off time T_OFF) may be adjusted to control the overall powerconsumption of display module 14A.

The time period between times T4 and T5 may form a duty period 381during which a light source other than the light source activated duringduty period 380 may be turned on to provide illumination light 38 tofLCOS display panel 40. A subsequent dark gap may begin at time T5, aswaveform 370 increases back to second drive voltage V_(ON). This cyclemay continue for each of the fields in the image frame to be displayed.The time period between times T4 and T5 may sometimes be referred to asfield period 378.

As shown in FIG. 27 , waveform 372 may remain at second drive voltageV_(ON) after time T0 and until time T2. By driving fLCOS display panel40 at second drive voltage V_(ON) during a portion of duty period 380(e.g., between times T1 and T2), fLCOS display panel 40 may reflect someof the illumination light 38 incident during duty period 380 (as imagelight 22). This may allow fLCOS display panel 40 to produce image light22 at a higher gray level when driven by waveform 372 than when drivenby waveform 370, for example.

The example of FIG. 27 is merely illustrative. In general, any desiredfLCOS drive voltage waveforms may be used to drive fLCOS display panel40 to produce any desired pixel values of any desired colors in imagelight 22. If desired, the optical performance of fLCOS display panel 40may be optimized by overdriving or underdriving the fLCOS drive voltageprovided to fLCOS display panel 40. The example of FIG. 27 in which thedrive voltage waveform follows a reset-based driving scheme is merelyillustrative. In another suitable arrangement, a reset-less drivingscheme may be used (e.g., there may not be dark gaps between each of thecolor fields and, if desired, an inverse waveform pattern may be usedafter each waveform pattern for charge balancing).

FIG. 28 is a timing diagram showing one example of how fLCOS displaypanel 40 may be overdriven to optimize optical performance. As shown inFIG. 28 , fLCOS display panel 40 may be driven using fLCOS drive voltagewaveform (curve) 392. Curve 390 of FIG. 28 plots the correspondingreflectance of fLCOS display panel 40 when driven using fLCOS drivevoltage waveform 392.

In the example of FIG. 28 , fLCOS drive voltage waveform 392 has fourpossible voltage levels (e.g., a first drive voltage level V1, a seconddrive voltage level V2, a third drive voltage level V3, and a fourthdrive voltage level V4). First drive voltage level V1 may be less thansecond drive voltage level V2, second drive voltage level V2 may be lessthan a voltage level of zero, third drive voltage level V3 may begreater than a voltage level of zero, and fourth drive voltage level V4may be greater than third drive voltage level V4. This example is merelyillustrative. In general, fLCOS drive voltage waveform 392 may have anydesired number of possible voltage levels of any desired magnitudes. Inone suitable arrangement that is sometimes described herein as anexample, first drive voltage level V1 may be -1.8 V, second drivevoltage level V2 may be -1.5 V, third drive voltage level V3 may be 1.5V, and fourth drive voltage level V4 may be 1.8 V. Other drive voltagelevels may be used if desired.

As shown by fLCOS drive voltage waveform 392, when fLCOS display panel40 is not being overdriven, fLCOS drive voltage waveform 392 may includesquare wave pulses such as square wave pulse 396 (e.g., where the fLCOSdrive voltage rises from second voltage level V2 to third voltage levelV3 at time TC and falls back to second voltage level V2 at time TD).Square wave pulse 396 may produce a corresponding spike in thereflectance of fLCOS display panel 40 from a reflectance of zero to areflectance of R (e.g., a value greater than 0 and less than 1.0), asshown by curve 390.

In order to overdrive fLCOS display panel 40, control circuitry 16 maydrive fLCOS display panel 40 using a non-square wave fLCOS drive voltagewaveform, such as an fLCOS drive voltage waveform that includesnon-square wave pulses such as non-square wave pulse 394 of fLCOS drivevoltage waveform 392. For example, at time TA, fLCOS drive voltagewaveform 392 may increase from second voltage level V2 to fourth voltagelevel V4 (sometimes referred to herein as overdrive voltage level V4).If desired, at time TA’, fLCOS drive voltage waveform 392 may decreaseto third voltage level V3. At time TB, fLCOS drive voltage waveform 392may decrease to first voltage level V1. At time TB’, fLCOS drive voltagewaveform 392 may increase back to second voltage level V2.

Non-square wave pulse 394 of fLCOS drive voltage waveform 392 mayproduce a corresponding spike in the reflectance of fLCOS display panel40 from a reflectance of zero at time TA to a reflectance greater thanreflectance R at or near time TA′ (e.g., a reflectance at or near 1.0).In other words, overdriving fLCOS display panel 40 in this way may serveto increase the reflectance of fLCOS display panel 40 relative toscenarios where fLCOS display panel 40 is not overdriven, therebymaximizing the overall optical efficiency of display module 14A inproducing image light 22.

The example of FIG. 28 is merely illustrative. In practice, curve 392and non-square wave pulse 394 may have other shapes. In general, fLCOSdisplay panel 40 may be overdriven using any desired non-square wavefLCOS drive voltage waveform (e.g., a waveform having non-square wavepulses that reach an overdrive voltage level such as fourth voltagelevel V4). Another example of a non-square wave pulse 394 that may beused to overdrive fLCOS display panel 40 is shown by dashed curve 393 ofFIG. 28 . In this example, the fLCOS drive voltage rises to voltagelevel V5 at time TA, drops continuously to voltage level V3 betweentimes TA and TB, drops to voltage level V0 at time TB, and risescontinuously to voltage level V2 between times TB and TB′. Voltage levelV5 may be greater than 1.8 V (e.g., 2.0 V) and voltage level V0 may beless than -1.8 V (e.g., -2.0 V), as one example. The precise shape ofcurve 393 between times TA and TB and between times TB and TB′ may, forexample, be altered to optimize the performance of fLCOS display panel40. The example of FIG. 28 in which fLCOS display panel 40 is overdrivenis merely illustrative and, if desired, fLCOS display panel 40 may beunderdriven. Different non-square wave fLCOS drive voltage waveforms maybe used to drive fLCOS display panel 40 at different times (e.g.,depending on the operating conditions of display 14).

In practice, the optimal overdrive or underdrive waveform for fLCOSdisplay panel 40 may vary as the operating temperature of fLCOS displaypanel 40 changes over time. If desired, control circuitry 16 mayoverdrive or underdrive fLCOS display panel 40 based on the temperatureof display 14 (e.g., the temperature of fLCOS display panel 40). FIG. 29is a flow chart of illustrative steps that may be performed by controlcircuitry 16 (FIG. 2 ) in overdriving or underdriving fLCOS displaypanel 40 based on the temperature of display 14.

At step 400, control circuitry 16 may gather temperature sensor datausing one or more temperature sensors 19 in system 10 (FIG. 1 ). Ifdesired, control circuitry 16 may estimate the temperature of fLCOSdisplay panel 40 based on the gathered temperature sensor data (e.g.,using a temperature model for system 10). In scenarios where multipletemperature sensors 19 are used to gather temperature sensor data, thetemperature sensors may be placed at different locations across system10 if desired. Control circuitry 16 may also determine whether a triggercondition has been met before proceeding.

The trigger condition may be a predetermined change in the gatheredtemperature sensor data, may occur when the gathered temperature datareaches a threshold temperature level, may be based on the content ofthe image(s) to be displayed using fLCOS display panel 40, may be basedon a software call issued by one or more programs running on system 10,may be based on a user input provided by a user of system 10, etc. Oncethe trigger condition has been met, processing may proceed to step 404as shown by arrow 402.

At step 404, control circuitry 16 may identify a non-square wave fLCOSdrive voltage waveform with which to overdrive or underdrive fLCOSdisplay panel 40 based on the gathered temperature sensor data. Forexample, control circuitry 16 may identify a non-square wave fLCOS drivevoltage waveform that optimizes the optical performance (e.g.,reflectance) of fLCOS display panel 40 for its current temperature(e.g., as determined while processing step 400). If desired, controlcircuitry 16 may store predetermined (optimal) non-square wave fLCOSdrive voltage waveforms for different temperature values of fLCOSdisplay panel 40 (e.g., in a look-up table or other data structure) andmay identify the stored non-square wave fLCOS drive voltage waveformcorresponding to the current (e.g., estimated) temperature of fLCOSdisplay panel 40. The stored non-square wave fLCOS drive voltagewaveforms may be determined during the design, manufacture, assembly,testing, and/or calibration of system 10 if desired.

At step 406, control circuitry 16 may drive fLCOS display panel 40 usingthe non-square wave drive voltage waveform identified while processingstep 404. Driving fLCOS display panel 40 in this way may maximize thereflectance of fLCOS display panel 40 for the current operatingtemperature of the display panel, for example. Control circuitry 16 maycontinue to overdrive fLCOS display panel 40 for a predetermined timeperiod, until a new trigger condition is detected, for a predeterminednumber of frames, etc.

The example of FIG. 29 in which control circuitry 16 overdrives fLCOSdisplay panel 40 based on the temperature of display 14 is merelyillustrative. In another suitable arrangement, control circuitry 16 mayoverdrive or underdrive fLCOS display panel 40 based on frame historyinformation. FIG. 30 is a flow chart of illustrative steps that may beperformed by control circuitry 16 (FIG. 2 ) in overdriving orunderdriving fLCOS display panel 40 based on frame history information.

At step 410, control circuitry 16 may identify frame history informationfor fLCOS display panel 40. The frame history information may include,for example, information about the image frames that have beenpreviously displayed using fLCOS display panel 40. Control circuitry 16may also determine whether a trigger condition has been met beforeproceeding.

The trigger condition may be a predetermined change in the gatheredtemperature sensor data, may occur when the gathered temperature datareaches a threshold temperature level, may be based on the content ofthe image(s) to be displayed using fLCOS display panel 40, may be basedon a software call issued by one or more programs running on system 10,may be based on a user input provided by a user of system 10, etc. Inone suitable arrangement that is described herein as an example, thetrigger condition may occur when the previous image frame displayed wasfully on or fully off. Once the trigger condition has been met,processing may proceed to step 414 as shown by arrow 412.

At step 414, control circuitry 16 may identify a non-square wave fLCOSdrive voltage waveform with which to overdrive or underdrive fLCOSdisplay panel 40 based on the identified frame history information. Forexample, control circuitry 16 may identify a non-square wave fLCOS drivevoltage waveform that optimizes the optical performance (e.g.,reflectance) of fLCOS display panel 40 depending on the immediatelyprevious image frame(s) displayed by fLCOS display panel 40 (e.g., afirst fLCOS drive voltage waveform when the previous image frame wasfully on, a second fLCOS drive voltage waveform when the previous imageframe was fully off, etc.).

At step 416, control circuitry 16 may drive fLCOS display panel 40 usingthe non-square wave drive voltage waveform identified while processingstep 414. Driving fLCOS display panel 40 in this way may maximize thereflectance of fLCOS display panel 40 for the current operatingtemperature of the display panel, for example. Control circuitry 16 maycontinue to overdrive fLCOS display panel 40 for a predetermined timeperiod, until a new trigger condition is detected, for a predeterminednumber of frames, etc.

Overdriving fLCOS display panel 40 (e.g., using non-square wave fLCOSdrive voltage waveforms as identified while processing step 404 of FIG.29 or step 414 of FIG. 30 ) may, for example, serve to reduce theduration (width) of the dark gap of fLCOS display panel 40 relative toscenarios where fLCOS display panel 40 is driven using a square wavefLCOS drive voltage waveform. This may serve to further optimize powerconsumption in display module 14A, for example. Control circuitry 16 maytherefore sometimes be referred to herein as reducing the duration ofthe dark gap of fLCOS display panel 40 based on gathered temperaturesensor data or identified frame history information. The arrangement ofFIG. 30 may be combined with the arrangement of FIG. 29 if desired(e.g., control circuitry 16 may identify a non-square wave fLCOS drivevoltage that optimizes the optical performance of fLCOS display panel 40given both the current temperature of fLCOS display panel 40 and theframe history information of fLCOS display panel 40). If desired, thenon-square wave drive voltage waveform to use may be selected based onthe previous frame’s target reflectance state and temperatureinformation. For example, a look up table may modify the non-square wavedrive voltage waveform for the current frame based on any previousstate. As one example, if the previous frame was on for one-half theillumination field time, it would have a different non-square wave drivevoltage for the current frame when the previous frame was on for 98% ofthe illumination field time. In driving fLCOS panel 40, the percent on(duty cycle) during the illumination time may be selected to control thegrey level for the field.

Overdriving fLCOS display panel 40 may also serve to optimize theoptical performance of display module 14A by reducing the response timeof fLCOS display panel 40. FIG. 31 is a plot showing how overdrivingfLCOS display panel 40 may reduce the response time of fLCOS displaypanel 40 across a wide range of operating temperatures.

In the example of FIG. 31 , the horizontal axis plots the temperature offLCOS display panel 40 (e.g., in degrees Celsius) and the vertical axisplots the response time of fLCOS display panel 40 (e.g., inmicroseconds). Curve 420 plots the response time of fLCOS display panel40 when driven using square-wave fLCOS drive voltage pulses (e.g.,pulses such as pulse 396 of FIG. 28 ). As shown by curve 420, theresponse time of fLCOS display panel 40 may decrease as temperatureincreases.

Curve 422 plots the response time of fLCOS display panel 40 when (over)driven using non-square-wave fLCOS drive voltage waveform pulses havinga first peak voltage level (e.g., pulses such as pulse 394 of FIG. 28having a peak voltage given by fourth voltage level V4). Curve 424 plotsthe response time of fLCOS display panel 40 when (over) driven using anon-square-wave fLCOS drive voltage waveform pulses having a second peakvoltage level that is higher than the first peak voltage level. The peakvoltage level of the square-wave fLCOS drive voltage pulses associatedwith curve 420 may be 1.5 V, the first peak voltage level associatedwith curve 422 may be 1.65 V, and the second peak voltage levelassociated with curve 424 may be 1.8 V, as one example. In general, thefirst peak voltage level may be any desired voltage greater than 1.5 V(e.g., in scenarios where V_(ON) of FIG. 27 is 1.5 V), greater than 1.6V, greater than 1.7 V, greater than 1.8 V, etc.

As shown by curves 422 and 424, overdriving fLCOS display panel 40 mayserve to reduce the response time of fLCOS display panel 40 across alltemperatures. As shown by curve 424, overdriving fLCOS display panel 40with a non-square wave fLCOS drive voltage waveform having a greaterpeak voltage level may serve to further decrease the response time offLCOS display panel 40. In this way, overdriving fLCOS display panel 40may serve to further optimize the optical performance of display module14A by reducing the response time of fLCOS display panel 40 across awide range of operating temperatures. The example of FIG. 31 is merelyillustrative. In practice, curves 420, 422, and 424 may have othershapes.

In accordance with an embodiment, a display system is provided thatincludes illumination optics that produce linear polarized illuminationlight, the illumination optics include: a light emitter configured toemit unpolarized light, and polarization recycling structures configuredto receive the unpolarized light and configured to output the linearpolarized illumination light based on the unpolarized light; aferroelectric liquid crystal on silicon (fLCOS) panel configured toproduce image light by modulating the linear polarized illuminationlight using image data; and a waveguide configured to propagate theimage light.

In accordance with another embodiment, the polarization recyclingstructures include a reflective polarizer.

In accordance with another embodiment, the polarization recyclingstructures include a quarter wave plate optically interposed between thereflective polarizer and the light emitter.

In accordance with another embodiment, the reflective polarizer includesa wire grid polarizer.

In accordance with another embodiment, the light emitter includes alight emitting diode (LED) die on a reflector and contact layer and theillumination optics include a patterned silicon substrate on the LEDdie, the wire grid polarizer being at least partially separated from thepatterned silicon substrate by an air gap.

In accordance with another embodiment, the reflective polarizer includesa reflective polarizer film.

In accordance with another embodiment, the light emitter includes alight emitting diode (LED) die on a reflector and contact layer and theillumination optics include a patterned silicon substrate on the LEDdie, the reflective polarizer film being separated from the patternedsilicon substrate by an air gap.

In accordance with another embodiment, the illumination optics include aglass layer overlapping the LED die and the reflective polarizer film iscoupled to the glass layer using adhesive.

In accordance with another embodiment, the LED die is mounted to aceramic substrate and the illumination optics include a spacer thatcouples the ceramic substrate to the glass layer.

In accordance with another embodiment, the illumination optics includean additional light emitter configured to emit unpolarized light and thepolarization recycling structures receive light from both the lightemitter and the additional light emitter.

In accordance with another embodiment, the illumination optics include asapphire substrate overlapping both the light emitter and the additionallight emitter and the illumination optics include a spacer thatseparates the polarization recycling structures from the sapphiresubstrate by an air gap.

In accordance with another embodiment, the illumination optics include acondenser lens and the polarization recycling structures are opticallyinterposed between the condenser lens and the light emitter.

In accordance with another embodiment, the polarization recyclingstructures are layered on a planar surface of the condenser lens.

In accordance with an embodiment, a display system is provided thatincludes a prism configured to output illumination light; a first lightsource that includes a first light emitting diode (LED) die configuredto emit unpolarized light of a first color and that includes firstpolarization recycling structures optically interposed between the firstLED die and the prism, the first polarization recycling structures areconfigured to output linearly polarized light of the first color; asecond light source that includes a second LED die configured to emitunpolarized light of a second color and that includes secondpolarization recycling structures optically interposed between thesecond LED die and the prism, the second polarization recyclingstructures are configured to output linearly polarized light of thesecond color and the prism includes: a first partial reflectorconfigured to reflect the linearly polarized light of the first color,and a second partial reflector configured to reflect the linearlypolarized light of the second color, the illumination light includes thelinearly polarized light of the first color and the linearly polarizedlight of the second color.

In accordance with another embodiment, the first partial reflectorincludes a first material interface that configures the first partialreflector to reflect the linearly polarized light of the first colorwhile transmitting light of the second color, the second partialreflector includes a second material interface that configures thesecond partial reflector to reflect the linearly polarized light of thesecond color while transmitting light of the first color, the displaysystem includes a third light source that includes a third LED dieconfigured to emit unpolarized light of a third color that is differentfrom the first and second colors, and the first and second partialreflectors are configured to transmit light of the third color.

In accordance with another embodiment, the first polarization recyclingstructures include a first reflective polarizer and the secondpolarization recycling structures include a second reflective polarizer.

In accordance with another embodiment, the first polarization recyclingstructures include a first quarter waveplate optically interposedbetween the first reflective polarizer and the first LED die and thesecond polarization recycling structures include a second quarterwaveplate optically interposed between the second reflective polarizerand the second LED die.

In accordance with another embodiment, the display system includes aferroelectric liquid crystal on silicon (fLCOS) display panel configuredto reflect the illumination light as image light; and a waveguideconfigured to propagate the image light.

In accordance with an embodiment, a display system is provided thatincludes a light emitting diode (LED) die configured to emit unpolarizedlight; a reflective polarizer configured to transmit a first linearpolarization of the unpolarized light and configured to reflect a secondlinear polarization of the unpolarized light back towards the LED die; aquarter waveplate optically interposed between the reflective polarizerand the LED die; and a ferroelectric liquid crystal on silicon (fLCOS)panel configured to produce image light based on the first linearpolarization of the unpolarized light transmitted by the reflectivepolarizer.

In accordance with another embodiment, the display system includes awaveguide configured to propagate the image light via total internalreflection.

The foregoing is merely illustrative and various modifications can bemade to the described embodiments. The foregoing embodiments may beimplemented individually or in any combination.

What is claimed is:
 1. A display system comprising: illumination opticsthat produce linear polarized illumination light, wherein theillumination optics comprise: a light emitter configured to emitunpolarized light, and polarization recycling structures configured toreceive the unpolarized light and configured to output the linearpolarized illumination light based on the unpolarized light; aferroelectric liquid crystal on silicon (fLCOS) panel configured toproduce image light by modulating the linear polarized illuminationlight using image data; and a waveguide configured to propagate theimage light.
 2. The display system of claim 1, wherein the polarizationrecycling structures comprise a reflective polarizer.
 3. The displaysystem of claim 2, wherein the polarization recycling structures furthercomprise a quarter wave plate optically interposed between thereflective polarizer and the light emitter.
 4. The display system ofclaim 2, wherein the reflective polarizer comprises a wire gridpolarizer.
 5. The display system of claim 4, wherein the light emittercomprises a light emitting diode (LED) die on a reflector and contactlayer and wherein the illumination optics further comprise a patternedsilicon substrate on the LED die, the wire grid polarizer being at leastpartially separated from the patterned silicon substrate by an air gap.6. The display system of claim 2, wherein the reflective polarizercomprises a reflective polarizer film.
 7. The display system of claim 6,wherein the light emitter comprises a light emitting diode (LED) die ona reflector and contact layer and wherein the illumination opticscomprise a patterned silicon substrate on the LED die, the reflectivepolarizer film being separated from the patterned silicon substrate byan air gap.
 8. The display system of claim 7, wherein the illuminationoptics comprise a glass layer overlapping the LED die and wherein thereflective polarizer film is coupled to the glass layer using adhesive.9. The display system of claim 8, wherein the LED die is mounted to aceramic substrate and wherein the illumination optics comprise a spacerthat couples the ceramic substrate to the glass layer.
 10. The displaysystem of claim 1, wherein the illumination optics comprise anadditional light emitter configured to emit unpolarized light andwherein the polarization recycling structures receive light from boththe light emitter and the additional light emitter.
 11. The displaysystem of claim 10, wherein the illumination optics comprise a sapphiresubstrate overlapping both the light emitter and the additional lightemitter and wherein the illumination optics comprise a spacer thatseparates the polarization recycling structures from the sapphiresubstrate by an air gap.
 12. The display system of claim 1, wherein theillumination optics comprise a condenser lens and wherein thepolarization recycling structures are optically interposed between thecondenser lens and the light emitter.
 13. The display system of claim12, wherein the polarization recycling structures are layered on aplanar surface of the condenser lens.
 14. A display system comprising: aprism configured to output illumination light; a first light source thatincludes a first light emitting diode (LED) die configured to emitunpolarized light of a first color and that includes first polarizationrecycling structures optically interposed between the first LED die andthe prism, wherein the first polarization recycling structures areconfigured to output linearly polarized light of the first color; asecond light source that includes a second LED die configured to emitunpolarized light of a second color and that includes secondpolarization recycling structures optically interposed between thesecond LED die and the prism, wherein the second polarization recyclingstructures are configured to output linearly polarized light of thesecond color and wherein the prism comprises: a first partial reflectorconfigured to reflect the linearly polarized light of the first color,and a second partial reflector configured to reflect the linearlypolarized light of the second color, wherein the illumination lightincludes the linearly polarized light of the first color and thelinearly polarized light of the second color.
 15. The display system ofclaim 14, wherein the first partial reflector comprises a first materialinterface that configures the first partial reflector to reflect thelinearly polarized light of the first color while transmitting light ofthe second color, wherein the second partial reflector comprises asecond material interface that configures the second partial reflectorto reflect the linearly polarized light of the second color whiletransmitting light of the first color, wherein the display systemfurther comprises a third light source that includes a third LED dieconfigured to emit unpolarized light of a third color that is differentfrom the first and second colors, and wherein the first and secondpartial reflectors are configured to transmit light of the third color.16. The display system of claim 14, wherein the first polarizationrecycling structures comprise a first reflective polarizer and whereinthe second polarization recycling structures comprise a secondreflective polarizer.
 17. The display system of claim 16, wherein thefirst polarization recycling structures comprise a first quarterwaveplate optically interposed between the first reflective polarizerand the first LED die and wherein the second polarization recyclingstructures comprise a second quarter waveplate optically interposedbetween the second reflective polarizer and the second LED die.
 18. Thedisplay system of claim 17, further comprising: a ferroelectric liquidcrystal on silicon (fLCOS) display panel configured to reflect theillumination light as image light; and a waveguide configured topropagate the image light.
 19. A display system comprising: a lightemitting diode (LED) die configured to emit unpolarized light; areflective polarizer configured to transmit a first linear polarizationof the unpolarized light and configured to reflect a second linearpolarization of the unpolarized light back towards the LED die; aquarter waveplate optically interposed between the reflective polarizerand the LED die; and a ferroelectric liquid crystal on silicon (fLCOS)panel configured to produce image light based on the first linearpolarization of the unpolarized light transmitted by the reflectivepolarizer.
 20. The display system of claim 19, further comprising awaveguide configured to propagate the image light via total internalreflection.